Ukrainian forces fought off Russian troops in the capital Kyiv on Friday as President Volodymyr Zelenskyy defied calls by his Russian counterpart Vladimir Putin for him to be overthrown.
Mr Putin unleashed a full-scale invasion on Thursday that has forced more than 50,000 people to flee Ukraine in just 48 hours.
NATO said on Friday it was deploying its rapid response forces to bolster defences on its eastern flank, as fears mounted of a new Cold War in Europe.
In his latest televised address on Friday, Mr Putin described the Ukrainian government as "terrorists" and "a gang of drug addicts and neo-Nazis".
"Take power in your own hands," he urged the Ukrainian military.
Mr Zelenskyy responded by posting a self-shot video on social media of himself on a Kyiv street, vowing to stay and defend the capital.
"We're all here. Our military is here. Citizens in society are here. We're all here defending our independence, our country, and it will stay this way," Mr Zelenskyy said outside the presidency building.
Mr Zelenskyy, who is Jewish, had earlier evoked Nazi Germany's 1941 invasion and praised his people for "demonstrating heroism" as Russian forces advanced towards the capital.
The US-led NATO alliance said Friday that Ukrainian forces were putting up resistance against the Russian forces.
"The Ukrainian forces are fighting bravely and are actually able to inflict damage on the invading Russian forces," NATO chief Jens Stoltenberg said after an emergency summit.
He added that the alliance has 100 jets and more than 120 ships on alert and that additional forces were being sent to eastern member states.
"We are deploying elements of the NATO Response Force on land, at sea, and in the air to further strengthen our posture and to respond quickly to any contingency," Mr Stoltenberg said.
Earlier on Friday, small arms fire and explosions were heard in Kyiv's northern district of Obolonsky as what appeared to be an advance party of Russia's invasion force left a trail of destruction.
Russia began a large-scale attack on Ukraine, with explosions reported in multiple cities and far outside the restive eastern regions held by Russian-backed rebels. Credit: Chris McGrath/Getty Images
In contrast, the city centre felt like a ghost town.
Intersections around the government district were manned by green armoured vehicles and machine-gun toting soldiers in balaclavas.
Sirens wailed at jarring intervals throughout the day. Booms echoed across the deserted streets.
Armed volunteers have joined a call to help the Ukrainian effort. Source: AFP / DANIEL LEAL/AFP via Getty Images
On Friday, Ukrainian forces reported fighting with Russian armoured units in two locations between 40-80 kilometres north of Kyiv.
Russian troops were also approaching the capital from the northeast and east, Ukraine's military said.
The Ukrainian defence ministry told civilians to resist.
"We urge citizens to inform us of troop movements, to make Molotov cocktails, and neutralise the enemy," it said.
Ukraine said 137 people, including soldiers and civilians, have been killed so far.
The Ukrainian defence ministry said that 2,800 Russian soldiers had been killed, without providing evidence, and Moscow has yet to give a report on casualties.
Russian Foreign Minister Sergei Lavrov said Moscow was ready to talk but only if Ukraine's armed forces "lay down their arms", adding that "nobody intends to occupy Ukraine".
A Kremlin spokesman meanwhile said Mr Putin was ready to send a delegation to Belarusian capital Minsk "for talks with a Ukrainian delegation".
On Friday evening, the UN said that more than 50,000 Ukrainians had fled the country in the past two days, calling for "safe unimpeded access" for aid operations.
Streams of people in cars and on foot were seen crossing into Hungary, Poland and Romania while hundreds camped out in a train station in the Polish border city of Przemysl.
Russia has demanded that Ukraine drop its ambition to join NATO and has called for the Western military alliance to scale back its presence in Eastern Europe.
As Mr Putin massed over 150,000 troops on Ukraine's borders, Western allies had initially imposed some sanctions on Russia in an effort to stop an invasion.
The European Union then moved to impose "massive" sanctions on Russia's energy and finance sectors on Thursday.
The EU's foreign policy chief Josep Borrell said Friday it had added Mr Putin and Mr Lavrov to the bloc's list of sanctions, along with the remaining members of the Russian parliament.
Despite Mr Zelenskyy expressly calling on Western allies to expel Moscow from the SWIFT banking transfer system, numerous EU countries including Germany, Hungary and Italy have been reluctant over fears Russia could cut off gas supplies.
There was also a response in the cultural and sporting worlds, with UEFA deciding that Paris will host this season's Champions League final instead of Saint Petersburg.
Formula One also said it was cancelling the Russian Grand Prix, while the International Olympic Committee called on all sports federations worldwide to cancel events in Russia.
Russia was also barred from participating in this year's Eurovision Song Contest.
