In an address ahead of the invasion, the Russian president warned of the "greatest consequences in history" for anyone who interfered.
Russian troops marched across Ukraine on Thursday, with missile strikes and explosions reported throughout the country.
Ukrainian president Volodymyr Zelenskyy said his citizens were listening to the sound of a new Iron Curtain falling, after Moscow mounted a massed assault by land, sea and air in the biggest attack on a European state since the Second World War.
Zelenskyy warned that other European countries may be next.
Western leaders have reacted in horror to the invasion.
Boris Johnson has vowed that Britain “cannot and will not just look away” and pledged to unite with allies to respond with sanctions designed to “hobble the Russian economy”.
Late on Thursday, he announced the government’s largest ever package of economic sanctions against Russia including an immediate asset freeze on all major Russian banks.
However, the UK prime minister has previously faced criticism for not being tougher with Putin in the run-up to war with many MPs saying the sanctions outlined earlier this week are not enough.
One senior Conservative MP, David Davis, called on the UK to retaliate with direct military involvement to prevent further escalation.
"Whether we like it or not Putin has effectively declared war on the West," Davis, a former Cabinet minister, said.
"He will view confining our actions to Article 5 measures as legalistic weakness."
Article 5 is a key part of the Nato alliance which states that an attack against one member state is considered as an attack against all 30 countries.
While Davis said it was "far too late" for the UK to put troops on the ground in Ukraine, he said "it is not too late to provide air support to the Ukrainian army which may neutralise Putin's overwhelming armoured superiority".
He added: "There are no zero-risk options. If we do not act militarily, then Nato will be significantly weakened and we must fear for the safety of every state that borders Russia...
"Ultimately it is a trade-off between two outcomes - whether we defend the democratic rights of more than 40 million Ukrainians or fail to act now and be forced to act later but at a possible greater cost."
On Thursday in an address to the nation, Johnson called Putin "the Russian dictator" and said the UK supported Ukraine's right to sovereignty - however, he stopped short of saying the UK would send military support.
"It's an attack on democracy and freedom - in eastern Europe and around the world," he said.
"This crisis is about the right of a free, sovereign, independent people to choose their own future. That is a right the UK will always defend."
James Cleverly, minister of state for Europe and North America, emphasised that while Ukraine was a "good friend" it is "not a member state of Nato".
"Nato's a defensive organisation, and our responsibility is to support our Nato allies - which is why we have sent troops and equipment to the eastern Nato allies," he said.
"And, whilst Ukraine is not a Nato state, we will continue to support them in their self-defence against this Russian attack."
On Thursday afternoon, Lithuania – a Nato state bordering Ukraine – announced it had declared a state of emergency in response Russia's invasion.
General Sir Richard Sherriff, Britain’s former top Nato commander, has called the Ukraine situation the most perilous in Europe for decades.
He said that while the war would initially be between Ukraine and Russia, the possibility that Nato could get involved is “very real”.
He told the BBC: “Nato has got to man the ramparts now", adding that the West has to “assume the worst” when it comes to Russia’s nuclear strategy.
To justify Russian aggression, Putin has claimed Ukraine is a threat to Moscow, that the country is controlled by neo-Nazis and that it is committing genocide in the east of the country.
Western leaders have ridiculed these claims.
On Friday afternoon, Putin hinted that he was open to talks with Ukraine, but required demilitarisation first. He also repeated false claims about the country being run by Nazis.
"I would like to address the Ukrainian army men, do not let these nationalists use you, your women, your children, your elderly, use them as human shields," he said.
"It will be so much easier to talk to you, to negotiate with you than with this gang of Nazis that have captured Kyiv."
Watch: Russia launches full-scale attack on Ukraine as Putin warns U.S. and NATO