MOSCOW — Russian President Vladimir Putin on Thursday announced a military operation in Ukraine and warned other countries that any attempt to interfere with the Russian action would lead to "consequences you have never seen."

He said the attack was needed to protect civilians in eastern Ukraine — a claim the U.S. had predicted he would falsely make to justify an invasion.

In a televised address, Putin accused the U.S. and its allies of ignoring Russia's demand to prevent Ukraine from joining NATO and offer Moscow security guarantees. He said Russia doesn't intend to occupy Ukraine but will move to "demilitarize" it and bring those who committed crimes to justice.

As Putin spoke before dawn, big explosions were heard in Kyiv, Kharkiv, Odessa and other cities across Ukraine.

U.S. President Joe Biden in a written statement condemned the "unprovoked and unjustified attack" on Ukraine and he promised that the U.S. and its allies "will hold Russia accountable." Biden said he planned to speak to Americans on Thursday after a meeting of the Group of Seven leaders. More sanctions against Russia were expected to be announced Thursday.

A full-blown Russian invasion could cause massive casualties and topple Ukraine's democratically elected government. And the consequences of the conflict and resulting sanctions levied on Russia could reverberate throughout the world, affecting energy supplies in Europe, jolting global financial markets and threatening the post-Cold War balance on the continent.

As he unleashed the military action, Putin issued a stark warning to other countries not to meddle.

"I have a few words for those who could feel tempted to interfere with ongoing developments: whoever tries to impede us, let alone create threats for our country and its people must know that the Russian response will be immediate and lead to the consequences you have never seen in history," he said.

Putin urged Ukrainian servicemen to "immediately put down arms and go home."

Putin announced the military operation after the Kremlin said rebels in eastern Ukraine asked Russia for military assistance to help fend off Ukrainian "aggression," an announcement that the White House said was a "false flag" operation by Moscow to offer up a pretext for an invasion.

Putin's announcement came just hours after the Ukrainian president rejected Moscow's claims that his country poses a threat to Russia and made a passionate, last-minute plea for peace.

"The people of Ukraine and the government of Ukraine want peace," President Volodymyr Zelenskyy said in an emotional overnight address, speaking in Russian in a direct appeal to Russian citizens. "But if we come under attack, if we face an attempt to take away our country, our freedom, our lives and lives of our children, we will defend ourselves. When you attack us, you will see our faces, not our backs."

Zelenskyy said he asked to arrange a call with Putin late Wednesday, but the Kremlin did not respond.

In an apparent reference to Putin's move to authorize the deployment of the Russian military to "maintain peace" in eastern Ukraine, Zelensky warned that "this step could mark the start of a big war on the European continent."

"Any provocation, any spark could trigger a blaze that will destroy everything," he said.

He challenged the Russian propaganda claims, saying that "you are told that this blaze will bring freedom to the people of Ukraine, but the Ukrainian people are free."

At an emergency meeting of the U.N. Security Council called by Ukraine because of the imminent threat of a Russian invasion, members still unaware of Putin's announcement appealed to him to stop an attack. U.N. Secretary-General Antonio Guterres opened the meeting, just before the announcement, telling Putin: "Stop your troops from attacking Ukraine. Give peace a chance. Too many people have already died."

Anxiety about an imminent Russian offensive soared after Putin recognized the separatist regions' independence on Monday, endorsed the deployment of troops to the rebel territories and received parliamentary approval to use military force outside the country. The West responded with sanctions.

Late Wednesday, Ukrainian lawmakers approved a decree that imposes a nationwide state of emergency for 30 days starting Thursday. The measure allows authorities to declare curfews and other restrictions on movement, block rallies and ban political parties and organizations "in the interests of national security and public order."

The action reflected increasing concern among Ukrainian authorities after weeks of trying to project calm. The Foreign Ministry advised against travel to Russia and recommended that any Ukrainians who are there leave immediately.

Pentagon press secretary John Kirby said Wednesday the Russian force of more than 150,000 troops arrayed along Ukraine's borders is in an advanced state of readiness. "They are ready to go right now," Kirby said.

Early Thursday, airspace over all of Ukraine was shut down to civilian air traffic, according to a notice to airmen. A commercial flight tracking website showed that an Israeli El Al Boeing 787 flying from Tel Aviv to Toronto turned abruptly out of Ukrainian airspace before detouring over Romania, Hungary, Slovakia and Poland. The only other aircraft tracked over Ukraine was a U.S. RQ-4B Global Hawk unmanned surveillance plane, which began flying westward early Thursday after Russia put in place flight restrictions over Ukrainian territory.

Another wave of distributed-denial-of-service attacks hit Ukraine's parliament and other government and banking websites on Wednesday, and cybersecurity researchers said unidentified attackers had also infected hundreds of computers with destructive malware.

Officials have long said they expect cyberattacks to precede and accompany any Russian military incursion, and analysts said the incidents hew to a nearly two-decade-old Russian playbook of wedding cyber operations with real-world aggression.

Even before Putin's announcement, dozens of nations imposed sanctions on Russia, further squeezing Russian oligarchs and banks out of international markets.

Biden allowed sanctions to move forward against the company that built the Russia-to-Germany Nord Stream 2 gas pipeline and against the company's CEO.

Germany said Tuesday that it was indefinitely suspending the project, after Biden charged that Putin had launched "the beginning of a Russian invasion of Ukraine" by sending troops into the separatist regions. The pipeline is complete but has not yet begun operating.

Even before the Russian military attack on Ukraine began, the threat of war had shredded Ukraine's economy and raised the specter of massive casualties, energy shortages across Europe and global economic chaos.

European Union sanctions against Russia took effect, targeting several companies along with 351 Russian lawmakers who voted for a motion urging Putin to recognize the rebel regions and 27 senior government officials, business executives and top military officers.

The Russian Foreign Ministry has shrugged off the sanctions, saying that "Russia has proven that, with all the costs of the sanctions, it is able to minimize the damage."