Russian forces launched a major assault on Ukraine, firing missiles at cities and military targets.

The land, air and sea invasion began after a pre-dawn televised speech in which Russian President Vladimir Putin demanded that the Ukrainian military lay down their arms.

Initial reports point to casualties among Ukrainian civilians and soldiers, as well as Russian troops.

The Ukrainian leader said his country "will not give up its freedom".

"Russia has gone down the wrong path, but Ukraine is fighting back," President Volodymyr Zelensky tweeted.

Ukraine declared martial law and severed all diplomatic relations with Russia. She claims that weapons will be given to anyone who wants them.

In the capital Kiev, which has a population of nearly three million, warning sirens sounded as traffic queued to leave the city and crowds sought refuge in metro stations. "We don't understand what we have to do now," a woman called Svetlana told the BBC. "We are now going to a place where we can be safe and hopefully we can leave safely."

Mark, a 27-year-old sales executive, said he was ready to be called upon to fight. "We have no other solution," he said.

Several neighboring countries have started preparing to receive large numbers of refugees. Moldova alone said more than 4,000 people crossed the border from Ukraine.

Thursday's invasion follows weeks of escalating tensions as Russia has massed troops along Ukraine's borders.

The UK, European Union and other Western allies have promised to impose tough new sanctions to punish Moscow, but say they will not send troops.

“These hours are among the darkest Europe has known since World War II,” said Josep Borrell, EU foreign affairs chief.

Dozens of people were killed, including a dozen civilians. Six of them perished in an airstrike in Brovary, near the capital kyiv. A man was also killed in a shelling outside the big city of Kharkiv, in the northeast of the country.

A Ukrainian presidential adviser said more than 40 soldiers had died and many more were injured. Ukraine said it killed 50 Russian soldiers and shot down six Russian planes, but this information has not been verified.

What Russia Aimed For

Mr Zelensky said Russia had positioned nearly 200,000 troops and thousands of combat vehicles on Ukraine's borders ahead of Thursday's invasion.

Moscow first hit Ukraine's military infrastructure and border guard units.

Then Ukrainian forces said that Russian military vehicles had crossed the border near Kharkiv in the north, Luhansk in the east, Russian annexed Crimea in the south and Belarus as well. Belarus' authoritarian leader Alexander Lukashenko said his country's military was not involved but could be if necessary.

Russian tanks were then seen in the suburbs of Kharkiv, a city of 1.4 million people. Russian forces are also said to have landed by sea in the main Ukrainian port cities, namely Odesa, on the Black Sea, and Mariupol, on the Sea of ​​Azov.

Most of the fighting seems to be concentrated in the east of the country.

Residents of Kharkiv, Ukraine's second largest city, say the windows of apartment buildings rattle from the constant explosions caused by shelling from the Ukrainian army and Russian forces.

But clashes also took place around the capital kyiv in the north and in the ports of Odesa and Mariupol in the south.

The Ukrainian military said Boryspil International Airport in Kyiv was among the airfields that were bombed, along with headquarters and warehouses in major cities of Kyiv, Dnipro, Kharkiv and Mariupol.

Footage verified by the BBC shows missiles hitting an airport in the western city of Ivano-Frankivsk.

Russia claimed to have destroyed more than 70 military targets.

"Unprovoked and unjustified".

The Russian leader launched the "special military operation" by repeating a number of baseless claims he made this week, including claiming that Ukraine's democratically elected government was responsible for eight years of genocide.

He said the goal was the demilitarization and "denazification" of Ukraine. A few hours earlier, the Ukrainian president had asked how a people who lost eight million of their citizens fighting the Nazis could support Nazism. “How could I be a Nazi?” replied Mr. Zelensky, himself a Jew.

Mr Putin also warned that any outside power intervening on Ukraine's behalf would face an "instant" response.

Neighboring countries have responded to the crisis.

In the Baltic Republic of Estonia, which borders Russia, Prime Minister Kaja Kallas said a number of NATO allies sharing borders with Russia had agreed to launch consultations under Article 4 NATO. Under the Defensive Alliance Treaty, NATO can be reunited if a member fears that its independence or territory may be threatened.

"Russia's widespread aggression is a threat to the whole world and to all NATO countries," she said.

As cars lined up at the Ukraine-Moldova border, the country's pro-EU President Maia Sandu said she was declaring a state of emergency and was ready to help tens of thousands of Ukrainians. Lithuanian President Gitanas Nauseda also said he was signing a state of emergency which must be approved by parliament.

"President Putin, in the name of humanity, bring your troops back to Russia," said UN Secretary General António Guterres.

Ukraine's Western allies have repeatedly warned that Russia is about to invade the country, despite repeated denials from Moscow. The United States, the European Union, the United Kingdom and Japan have imposed sanctions on key Russians, Russian banks and MPs who supported this initiative.

In a televised address, British Prime Minister Boris Johnson said "Vladimir Putin's hideous and barbaric enterprise must end in failure".

Addressing the Russians, he said: "I can't believe this is being done in your name, or that you really want the pariah status this will bring to Putin's regime." He told Ukrainians that the UK was "on your side".

US President Joe Biden has said the world will hold Russia to account. He is expected to address Americans on Thursday about the consequences Russia will face.

French President Emmanuel Macron said the attack would have "profound and lasting consequences on our lives".