Watch Boris Johnson's address to the nation in full
In an address to the nation, the prime minister said: "Our mission is clear diplomatically, politically, eventually military, this hideous and barbaric venture of Vladimir Putin must end in failure."
He added that the world could not allow the freedom of Ukraine to be "snuffed out" and "we cannot and will not just look away".
The Russian invasion was launched overnight, with President Putin telling the West any interference would lead to "consequences you have never seen."
Ahead of launching his invasion, the strong-arm leader warned Ukrainians to "lay down your arms immediately and go home".
"All Ukrainian soldiers who comply with this requirement can freely leave the area of military action and return to their families," Putin said.
The assault on this sovereign nation has finally come - and everything has changed, as ITV News Global Security Editor Rohit Kachroo reports
Mr Johnson said: "This act of wanton and reckless aggression is an attack not just on Ukraine, it's an attack on democracy and freedom in eastern Europe and around the world."
The PM criticised the the Russian president for having "unleashed war in our European continent", attacking Ukraine "without any provocation and without any credible excuse".
"Innumerable missiles and bombs have been raining down on an entirely innocent population," Mr Johnson said, adding that the invasion was land, sea and air based.
Downing Street said the UK is providing "defensive military capability" to Ukraine.
The PM's official spokesman said "Our expectation is that the Ukrainian people will fight, will engage with Russian forces ... we are providing military, defensive military capability."
Foreign Secretary Liz Truss held a meeting with Russia's ambassador to London during the invasion in which she told him his country's "government had repeatedly lied about having no plans to invade Ukraine, and its unprovoked aggression had made it an international pariah".
The Foreign Office said she "condemned Russia’s outrageous attack on Ukraine as a clear breach of international law" and "reiterated there would be severe sanctions in retribution for the invasion".
"She added that Russia should expect a long, protracted conflict that would inflict a huge human, economic and political cost on the Russian government. She said allies and partners are united in support for Ukraine, providing economic and defensive support."
Mr Johnson said he will also speak to leaders of G7 countries Canada, France, Germany, Italy, Japan, and the United States.
He spoke to German chancellor Olaf Scholz ahead of that meeting, in which he "underscored that western inaction or underreaction would have unthinkable consequences".
Downing Street said "the leaders agreed to stay in close contact and to discuss our response further in this afternoon’s meeting of G7 leaders.”
Ukrainians living in the UK gathered to protest outside Downing Street as the PM's speech was broadcast, displaying placards apparently urging Mr Johnson to "Save Ukraine from Putin's Red Terrorism".
Nato announced it would be beefing up its "air, land and sea forces on its eastern flank near Ukraine and Russia".
Former foreign secretary Jeremy Hunt said the West needed to be "strategic" in its response to the invasion because "it's not just Russia, we have China that is also committed to upending the international order".
"This is the first time for very many years that a major power has decided, basically just to grab another country - the kind of thing that happened in the 19th Century and some of the darkest days of the 20th Century," he said.
The MP urged Mr Johnson to do "a lot more" beyond the sanctions already imposed by stopping the purchase of Russian gas and blocking Russian millionaires from using London to launder dirty money.
He added that Putin is a "bully" and the UK must ensure the "price he pays for this act is too high for Russia to bear".
Labour leader Sir Keir Starmer called for the "hardest possible sanctions" against Mr Putin's "bandit rule".
Ukraine calls for the UK to issue the following sanctions on Russia:
Ban energy trade with Russia - stop buying Russian oil and gas
Ban foreign investment in Russia and prohibit Western government pension funds from investing in Russian assets
Sanction all secondary trading in Russian debt and equities - public and private sector including existing holdings
Sanction the Russian Central Bank, Russian state owned and private banks
Sanction all ruble transactions with Western banks
Withdraw Russia out of bond and equity investment indices
Block Russia from Visa and MasterCard payment system
Freeze Russian foreign assets, sanction Russian Direct Investment Fund
Expand personal sanctions to all those in Western list of sanctions and their families - asset freeze, visa cancelations, passport revocation and repatriation back to Russia
All UK airlines have been instructed to avoid Ukrainian airspace after the Department for Transport announced a ban on flights between the UK and Ukraine.
Overnight Mr Johnson said President Putin has "chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine".
Mr Johnson condemned the military action and is expected to announce further sanctions, as he promised to respond “decisively” to the Russian onslaught.
The UK has sent military aid to Ukraine ahead of the invasion and has soldiers stationed in eastern Europe but previously said it would not engage in combat.
Asked if the UK could join the fighting, Foreign Office Minister James Cleverly told ITV News "Ukraine is a good friend but they are not a member state of Nato".
He said the UK will help Ukraine defend itself but suggests the British military would be unable to fight.
European Commission President Ursula von der Leyen said Putin is "bringing war back to Europe".
"Once again in the centre of Europe, innocent women, men and children are dying”, she said.
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