KYIV, Ukraine Russia launched a full-scale invasion of Ukraine on Thursday, unleashing airstrikes on cities and military bases, and sending in troops and tanks from three sides in an attack that could rewrite the global post-Cold War security order. Ukraines government pleaded for help as civilians piled into trains and cars to flee.

Military forces battling Russians on multiple fronts suffered dozens of casualties.

Before dawn Friday, more explosions were heard in Ukrainian capital, Kyiv, as Russian forces continued their assault. Several journalists published footage that appeared to show missile strikes on the capital.

A Ukrainian government official, Anton Gerashchenko, said Kyiv was being targeted by cruise or ballistic missiles, CNN reported .

On Friday morning, Zelenskyy made another appeal directly to Russians watching the invasion from home as Ukrainian officials braced for an attack on Kyiv.

Tonight, you began bombing residential areas in the hero city of Kyiv. This is like 1941, the Ukrainian president said, according to a translation by the Financial Times. I want to tell all Russian citizens who are coming out to protest: We hear you, you heard us, you started to believe us. Fight for us. Fight the war.

A day earlier, Russian President Vladimir Putin ignored global condemnation and cascading new sanctions as he unleashed the largest ground war in Europe since World War II and chillingly referred to his countrys nuclear arsenal. He threatened any nation trying to interfere with consequences you have never seen, as a once-hoped-for diplomatic resolution now appeared impossible.

Ukrainian forces sought to fend off a Russian barrage of land- and sea-based missiles, an attack that one senior U.S. defense official described as the first salvo in a likely multi-phase invasion aimed at seizing key population centers, decapitating Ukraines government and installing a new one. Already, Ukraine officials said they had lost control of the decommissioned Chernobyl nuclear power plant , the scene of the worlds worst nuclear disaster.

Russia has embarked on a path of evil, but Ukraine is defending itself and wont give up its freedom, Ukrainian President Volodymyr Zelenskyy tweeted. His grasp on power increasingly tenuous, he pleaded Thursday for even more severe sanctions than the ones imposed by Western allies and ordered a full military mobilization that would last 90 days.

Zelenskyy said in a video address that 137 heroes, including 10 military officers, had been killed and 316 people wounded. The dead included all border guards on the Zmiinyi Island in the Odesa region, which was taken over by Russians.

Zelenskyy said in the address that Russian saboteurs had entered Kyiv and said intelligence found the enemy has identified me as the number one target.

They want to destroy Ukraine politically by destroying the head of state, he said, adding that his family was the number two target.

President Joe Biden announced new sanctions against Russia, saying that Putin chose this war and that his country would bear the consequences. Other nations also announced sanctions or said they would shortly.

Fearing a Russian attack on the capital, thousands of people went deep underground as night fell, jamming Kyivs subway stations.

At times it felt almost cheerful. Families ate dinner. Children played. Adults chatted. People brought sleeping bags, dogs, crossword puzzles anything to alleviate the waiting and the long night ahead.

But the exhaustion was clear on many faces. And the worries.

Nobody believed that this war would start and that they would take Kyiv directly, said Anton Mironov, waiting out the night in one of the old Soviet metro stations. I feel mostly fatigue. None of it feels real.

The invasion began early Thursday with a series of missile strikes, many on key government and military installations, quickly followed by a three-pronged ground assault. Ukrainian and U.S. officials said Russian forces were attacking from the east toward Kharkiv, Ukraines second-largest city; from the southern region of Crimea, which Russia annexed in 2014; and from Belarus to the north.

Zelenskyy, who had earlier cut diplomatic ties with Moscow and declared martial law, appealed to global leaders, saying that if you dont help us now, if you fail to offer a powerful assistance to Ukraine, tomorrow the war will knock on your door.

Though Biden said he had no plans to speak with Putin, the Russian leader did have what the Kremlin described as a serious and frank exchange with French President Emmanuel Macron.

Both sides claimed to have destroyed some of the others aircraft and military hardware, though little of that could be confirmed.

Hours after the invasion began, Russian forces seized control of the now-unused Chernobyl plant and its surrounding exclusion zone after a fierce battle, presidential adviser Myhailo Podolyak told The Associated Press.

The Vienna-based International Atomic Energy Agency said it was told by Ukraine of the takeover, adding that there had been no casualties or destruction at the industrial site.

The 1986 disaster occurred when a nuclear reactor at the plant 80 miles north of Kyiv exploded, sending a radioactive cloud across Europe. The damaged reactor was later covered by a protective shell to prevent leaks.

Alyona Shevtsova, adviser to the commander of Ukraines ground forces, wrote on Facebook that staff members at the Chernobyl plant had been taken hostage. The White House said it was outraged by the reports of hostage-taking.

The chief of the NATO alliance, Jens Stoltenberg, said the brutal act of war shattered peace in Europe. He joined a chorus of world leaders decrying an attack that could cause massive casualties and topple Ukraines democratically elected government. The conflict shook global financial markets: Stocks plunged and oil prices soared amid concerns that heating bills and food prices would skyrocket.

Condemnation came not only from the U.S. and Europe but also from South Korea, Australia and beyond and many governments readied new sanctions. Even friendly leaders like Hungarys Viktor Orban sought to distance themselves from Putin.

British Prime Minister Boris Johnson said he aimed to cut off Russia from the U.K.s financial markets as he announced sanctions, freezing the assets of all large Russian banks and planning to bar Russian companies and the Kremlin from raising money on British markets.

Now we see him for what he is a bloodstained aggressor who believes in imperial conquest, Johnson said of Putin.

The U.S. sanctions will target Russian banks, oligarchs, state-controlled companies and high-tech sectors, Biden said, but they were designed not to disrupt global energy markets. Russian oil and natural gas exports are vital energy sources for Europe.

Zelenskyy urged the U.S. and the West to go further and cut the Russians from the SWIFT system, a key financial network that connects thousands of banks around the world. The White House has been reluctant to immediately cut Russia from SWIFT, worried it could cause enormous economic problems in Europe and elsewhere in the West.

While some nervous Europeans speculated about a possible new world war, the U.S. and its NATO partners have shown no indication they would send troops into Ukraine, fearing a larger conflict. NATO reinforced its members in Eastern Europe as a precaution, and Biden said the U.S. was deploying additional forces to Germany to bolster NATO.

European authorities declared the countrys airspace an active conflict zone.

After weeks of denying plans to invade, Putin launched the operation on a country the size of Texas that has increasingly tilted toward the democratic West and away from Moscows sway. The autocratic leader made it clear earlier this week that he sees no reason for Ukraine to exist, raising fears of possible broader conflict in the vast space that the Soviet Union once ruled. Putin denied plans to occupy Ukraine, but his ultimate goals remain hazy.

Ukrainians were urged to shelter in place and not to panic.

Until the very last moment, I didnt believe it would happen. I just pushed away these thoughts, said a terrified Anna Dovnya in Kyiv, watching soldiers and police remove shrapnel from an exploded shell. We have lost all faith.

With social media amplifying a torrent of military claims and counter-claims, it was difficult to determine exactly what was happening on the ground.

Russia and Ukraine made competing claims about damage they had inflicted. Russias Defense Ministry said it had destroyed scores of Ukrainian air bases, military facilities and drones. It confirmed the loss of one of its Su-25 attack jets, blaming pilot error, and said an An-26 transport plane had crashed because of technical failure, killing the entire crew. It did not say how many were aboard.

Russia said it was not targeting cities, but journalists saw destruction in many civilian areas.

Ukraines health minister said 57 Ukrainians were killed in the invasion and 169 more were wounded. It was not clear how many were civilians, although earlier in the day it had said 40 soldiers had died.

Polands military increased its readiness level, and Lithuania and Moldova moved toward doing the same.

Putin justified his actions in an overnight televised address , asserting the attack was needed to protect civilians in eastern Ukraine a false claim the U.S. predicted he would make as a pretext for invasion. He accused the U.S. and its allies of ignoring Russias demands to prevent Ukraine from joining NATO and for security guarantees, saying the military action was a forced measure.

Anticipating international condemnation and countermeasures, Putin issued a stark warning to other countries not to meddle.

In a reminder of Russias nuclear might, he warned that no one should have any doubts that a direct attack on our country will lead to the destruction and horrible consequences for any potential aggressor.

Inside Russia, authorities moved swiftly to crack down on any critical voices. OVD-Info, a group that tracks political arrests, reported 1,620 people in 52 Russian cities had been detained for protesting the invasion, more than half of them in Moscow.

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Isachenkov and Litvinova reported from Moscow. Francesca Ebel in Kyiv; Angela Charlton in Paris; Geir Moulson and Frank Jordans in Berlin; Raf Casert and Lorne Cook in Brussels; Nic Dumitrache in Mariupol, Ukraine; Inna Varennytsia in eastern Ukraine; and Robert Burns, Matthew Lee, Aamer Madhani, Eric Tucker, Nomaan Merchant, Ellen Knickmeyer, Zeke Miller, Chris Megerian and Darlene Superville in Washington contributed.

HuffPost contributed to this report.