MOSCOW — President Vladimir V. Putin of Russia declared the start of a “special military operation” in Ukraine on Thursday, after months of speculation about Russia’s intentions as it massed tens of thousands of troops on Ukraine’s border.
Addressing his nation in a televised speech broadcast just before 6 a.m. Thursday, Mr. Putin said his goal was to “demilitarize” but not occupy the country.
Minutes later, large explosions were visible near Kharkiv, Ukraine’s second-largest city, and blasts were reported in Kyiv, the capital, and other parts of the country.
Ukraine’s Interior Ministry said that Russian troops had landed in Odessa and were crossing the border.
“The invasion has begun,” the ministry said in a statement.
Ukraine’s foreign minister, Dmytro Kuleba, said on Twitter that Mr. Putin had “started a full-scale war against Ukraine” and had begun shelling civilian cities.
“This is a war of aggression,” he wrote on Twitter. “Ukraine will defend itself and win. The world must act and stop Putin. It is time to act — immediately.”
Evoking the NATO bombing of Yugoslavia in 1999 and the American invasion of Iraq in 2003, Mr. Putin cast his action as a long-overdue strike against an American-led world order that he described as an “empire of lies.”
Even as he spoke, the United Nations Security Council held an emergency meeting imploring him not to invade.
Mr. Putin said he was acting after receiving a plea for assistance from the leaders of the Russian-backed separatist territories formed in eastern Ukraine in 2014 — a move that Western officials had predicted as a possible pretext for an invasion.
Mr. Putin also described the operation as a response to a “question of life or death” that he said Russia was facing as a result of the eastward expansion of the NATO alliance — which Ukraine has aspired to join.
“This is that red line that I talked about multiple times,” Mr. Putin said. “They have crossed it.”
The operation’s goal, Mr. Putin said, was “to defend people who for eight years are suffering persecution and genocide by the Kyiv regime,” citing the false accusation that Ukrainian forces had been carrying out ethnic cleansing in separatist regions of eastern Ukraine.
In bellicose language, Mr. Putin also issued what appeared to be a warning to other countries.
“Anyone who tries to interfere with us, or even more so, to create threats for our country and our people, must know that Russia’s response will be immediate and will lead you to such consequences as you have never before experienced in your history,” Mr. Putin said. “We are ready for any turn of events.”
In a statement, President Biden placed responsibility for the conflict squarely on Mr. Putin’s shoulders.
“President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” Mr. Biden said. “Russia alone is responsible for the death and destruction this attack will bring, and the United States and its allies and partners will respond in a united and decisive way. The world will hold Russia accountable.”
He added that he would address the American people on Thursday about “further consequences” the United States and its allies would impose on Russia.
On Wednesday, Ukraine had mobilized its reservists and declared a 30-day state of emergency as cyberattacks knocked out government institutions including Parliament, the Foreign Ministry and the cabinet of ministers.
In Kyiv, President Volodymyr Zelensky made an impassioned bid earlier in the day to spare his nation from war, appealing directly to the Russian people to remember their ties to Ukraine.
“Listen to the voice of reason,” Mr. Zelensky said in a nationally televised address early Thursday, adding that Kremlin propaganda painting Ukrainians as aggressors was a lie. “The people of Ukraine want peace, the authorities in Ukraine want peace.”
The West unveiled new sanctions targeting Mr. Putin’s inner circle, with threats of tighter measures if Russia escalated hostilities, but a senior Russian diplomat denigrated the idea that pressure would alter Russia’s course, suggesting that the sanctions would only create economic pain for the West.
In Washington on Wednesday, the Pentagon said that 190,000 Russian troops and separatist forces were poised along the Ukrainian border and that 80 percent of them were positioned for combat.
“They are ready to go,” John F. Kirby, the top Pentagon spokesman, told reporters.
Senator Mark Warner, the Virginia Democrat who leads the Senate Intelligence Committee, said Russia’s invasion of Ukraine had brought decades of general peace in Europe to an end.
“While there is still an opportunity for Russia to reverse course, we can no longer hold out hope that this standoff will be resolved peacefully,” Mr. Warner said. “Therefore, we must all, on both sides of the aisle and both sides of the Atlantic, work together to demonstrate to Putin that this aggression will not be allowed to go unpunished.”
Several hundred Russian mercenaries from the Wagner paramilitary group had arrived in the separatist regions of Donetsk and Luhansk even before Mr. Putin announced the military operation on Thursday, according to two senior European security officials.
The commandos, with experience fighting in Syria and Libya, were flown into Crimea, a peninsula annexed by Russia after sending in troops in 2014, and have since trickled into the rebel-held territories covertly in civilian clothes, the officials said.
Cyberattacks, another component of Russia’s assault on Ukraine, continued on Wednesday. The websites of various Ukrainian government institutions, including Parliament, the Foreign Ministry and the cabinet of ministers, crashed after a denial-of-service attack.
In Mr. Zelensky’s speech, delivered in Russian, he conceded that his appeal would probably not be heard in Russia, where the media is largely state controlled, and said that an attempt to call Mr. Putin directly was met with silence.
He tried to address the main accusations leveled against Ukrainians by the Kremlin. Ukrainians were not Nazis, he said, and his own grandfather had served in the Soviet Army throughout the war. They did not hate Russian culture, he said.
“We are different,” he said, “but that is not a reason to be enemies. We want to determine, build our future ourselves, peacefully, calmly and honestly.”
Speaking of the contested areas of Donetsk and Luhansk, Mr. Zelensky said he suspected that the region was foreign to most Russians.
“This is our land, our history,” he said. “What are you going to be fighting for and with whom?”
Mr. Zelensky changed tack toward the end, warning that Ukrainians would fight to repel any attack.
“We won’t attack, but we will defend ourselves,” he said. “By attacking, you will see our faces — not our backs, but our faces.”
Earlier, Ukraine’s Security Council declared a 30-day state of national emergency in response to the threat of a Russian invasion. The government urged Ukrainian citizens in Russia to leave. Russia, in turn, began withdrawing more diplomats from Ukraine.
Ukraine’s Parliament also began formally working on plans for a law that would allow civilians to own firearms, one day after President Zelensky called up military reservists to fight for their country before it disappears.
The list of sanctions imposed on Russia continued to grow, with European leaders expected to hold an emergency summit in Brussels on Thursday to discuss further steps. So far the sanctions have avoided steps that would harm Europe, like targeting the energy sector.
On Wednesday, the European Union announced sanctions on various high-profile Russian officials and media figures, including the defense minister, Sergei K. Shoigu, and Mr. Putin’s chief of staff, Anton Vaino.
Australia, Canada and Japan also unveiled sanctions. And the White House announced a second round of sanctions, on the company building the gas pipeline connecting Russia to Germany.
E.U. officials described their sanctions list — nearly 600 pages that included travel bans and asset freezes — as just a first step toward punishing those involved in Russia’s recognition on Monday of the so-called republics of Donetsk and Luhansk as independent states, a move that allowed their leaders to request Russian military assistance on Wednesday.
Two prominent individuals on the list were Maria Zakharova, the spokeswoman for the Foreign Ministry, and Margarita Simonyan, who leads the television network RT and has been a vocal cheerleader on social media for Russian intervention in Ukraine.
Yevgeny Prigozhin — a wealthy Russian businessman sometimes known as “Putin’s cook” because he first attracted the president’s attention through his catering business, and was later linked to the Wagner mercenary group — was included on the E.U. sanctions list alongside several members of his family.
Mr. Biden said on Wednesday he would issue economic sanctions on the company building the gas pipeline connecting Russia to Germany. The Biden administration warned of additional measures in the event Russia escalates an armed conflict, signaling that it is ready to impose a ban on exports of American technology that are vital to the Russian economy.
The warning came a day after the United States announced sanctions on two Russian banks and curbs on Russia’s sovereign debt, effectively cutting the country off from Western financing. Russia has amassed more than $600 billion in reserves in recent years to withstand such measures. Export controls would represent a significant expansion of the tools the United States is prepared to deploy to respond to further aggression.
Australia will impose travel bans and financial penalties on eight members of Russia’s National Security Council, Prime Minister Scott Morrison said Wednesday. It will also punish several Russian banks.
Prime Minister Justin Trudeau of Canada said Tuesday that his country would enact prohibitions on dealing with Russian sovereign debt and impose sanctions on two Russian banks. Canada will also penalize Russian lawmakers who voted to recognize the two regions.
In Japan, Prime Minister Fumio Kishida said Wednesday that the country would enact similar sanctions against Russia, including prohibiting it from issuing new sovereign bonds in Japanese markets. Most countries banned doing business with Donetsk and Luhansk, a largely symbolic gesture since the battered Soviet industrial areas have been isolated since 2014.
Russia’s ambassador to the United States responded defiantly, saying that the country was used to living under such sanctions from the West and that the new penalties would hurt global financial and energy markets as well as Americans.
“It is hard to imagine that there is a person in Washington who expects Russia to revise its foreign policy under a threat of restrictions,” the ambassador, Anatoly Antonov, said on Facebook.
Ukraine welcomed the measures but called for even tougher restrictions against Russia.
In Russia itself, celebrating Defenders of the Fatherland Day, a national holiday marking the founding of the Red Army, Mr. Putin reiterated his combative message. Russia’s demands for an equitable system of international security “remain unanswered,” he said, blaming military activity by the NATO bloc for what he described as “the difficult international situation.”
At a news conference late Tuesday, Mr. Putin declared dead the Minsk Protocol, an agreement reached by Russia, Ukraine and Western intermediaries in 2015 to end the fighting in eastern Ukraine and long considered the main vehicle for a diplomatic solution.
“The Minsk agreements do not exist anymore,” he told reporters. “Why abide by them if we have recognized the independence of these entities?”
Russia has effectively controlled those areas since March 2014. Late Wednesday, Russian state media published letters to Mr. Putin from the separatist leaders that cited the “friendship, cooperation and mutual support” treaties that they had signed with him on Monday. Both letters were dated Feb. 22.
American and European security officials have warned repeatedly that Moscow was planning a “false flag” operation, an invented attack by Ukraine to justify a Russian offensive.
“Kyiv is currently continuing to build up its military presence on the contact line while receiving extensive support, including military support, from the United States of America and other western countries,” one of the separatist leaders, Leonid Pasechnik of the Luhansk region, wrote in the letter to Mr. Putin. “In defiance of its international obligations, the regime in Kyiv is focused on a military solution to its conflict with the Luhansk People’s Republic.”
Reporting was contributed by Eric Schmitt and Julian E. Barnes from Washington; Valerie Hopkins from Kyiv, Ukraine; Matina Stevis-Gridneff from Brussels; Michael Schwirtz from Slaviansk, Ukraine; Christoph Koettl from New York; and Haley Willis from London.