The UK “cannot and will not just look away” at Russia’s “hideous and barbaric” attack on Ukraine, Boris Johnson has said.
The PM said President Vladimir Putin had launched a “vast invasion by land, by sea and by air” without provocation.
He said the UK and allies will launch a “massive package” of sanctions to “hobble” Russia’s economy.
The PM will outline the further economic sanctions against Russia in the Commons at 17:00 GMT.
In a pre-recorded TV statement, Mr Johnson stressed that Ukraine was “not some faraway country of which we know little”.
He added: “We have Ukrainian friends in this country, neighbours, co-workers. Ukraine is a country that for decades has enjoyed freedom and democracy and the right to choose its own destiny.”
He said the UK and the world could not allow that freedom “just to be snuffed out”.
As a result, the UK and its allies would agree a “massive package of economic sanctions” in a bid to “hobble” the Russian economy, Mr Johnson said, before warning that the West would need to cease its dependence on Russian oil and gas.
“Our mission is clear: diplomatically, politically, economically and eventually militarily, this hideous and barbaric venture of Vladimir Putin must end in failure,” he said.
Mr Johnson’s comments come after Russian forces launched an assault on Ukraine, crossing its borders and bombing military targets near big cities.
Ukraine announced that martial law – which means the military takes control temporarily – has been imposed across the whole of the country.
Traffic jams have built up as people fled the capital Kyiv.
Russian military vehicles are reported to have breached Ukraine’s border in a number of places, in the north, south and east, including from Belarus.
At least seven people are known to have been killed by Russian shelling, including civilians. A Ukrainian presidential adviser said that more than 40 soldiers had died and dozens more were wounded, but this has not been independently confirmed.
Addressing the Ukrainian people directly, Mr Johnson said: “In this moment of agony, we’re with you, we’re praying for you and your families and we are on your side.”
But he said if the months ahead were “grim and the flame of freedom burned low”, he firmly believed it would “blaze bright again in Ukraine”.
“Because for all his bombs and tanks and missiles, I don’t believe that the Russian dictator will ever subdue the national feeling of the Ukrainians and their passionate belief that their country should be free,” he added.
The prime minister also sought to reassure the British public, pledging to do “everything to keep our country safe” and work with allies “for however long it takes” to restore Ukraine’s sovereignty and independence.
“Because 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”.
Appealing directly to the Russian people, Mr Johnson said their president had launched a “tidal wave of violence against fellow Slavic people” before adding: “To the parents of Russian soldiers who will lose their lives, I cannot believe this is being done in your name.”
The PM is now speaking with leaders of the G7 group of wealthy nations about the situation. Earlier, he said on Twitter that the invasion was a “catastrophe for our continent”.
Labour leader Sir Keir Starmer said the UK should brace itself for the “economic pain” the country will experience by imposing sanctions against Russia.
In a televised address, Sir Keir said: “We will see economic pain as we free Europe from dependence on Russian gas and clean our institutions from money stolen from the Russian people.
“But the British public have always been willing to make sacrifices to defend democracy on our continent and we will again.”
Sanctions and assistance but no boots on the ground
Senior ministers in London are spending the next few hours speaking to their counterparts in the capitals of wealthy G7 nations.
The government intends to announce what is says is a severe sanctions package later this afternoon – when the prime minister will address Parliament. The government wants it to be done in line with allies.
After criticism the initial sanctions package did not go far enough, ministers made clear they had more waiting to be used at the right time.
It is likely that will involve travel bans and asset freezes on more individuals and sanctions on more banks. It could also extend to key Russian industries like technology, energy and chemicals.
Ministers have also been discussing ways to restrict Russian access to UK financial systems. Labour wants Russia banned from the Swift system for international payments.
The UK is also planning to provide military assistance to Ukraine – but that will not involve boots on the ground.
Foreign Secretary Liz Truss has summoned the Russia’s UK ambassador Andrei Kelin, for the second time this week, to explain the “illegal, unprovoked invasion of Ukraine”.
Meanwhile the head of the Western defensive alliance Nato, Jens Stoltenberg, condemned the invasion as a “blatant violation” of international law and said he is calling a virtual summit of alliance leaders on Friday to discuss the “serious threat” to security in the region.
“This is a grave moment for the security of Europe. Russia’s unprovoked and unjustified attack on Ukraine is putting countless lives at risk,” he said.
In a pre-dawn TV statement on Thursday, President Putin said Russia did not plan to occupy Ukraine, but demanded its soldiers lay down their weapons, before warning that Moscow’s response would be “instant” if anyone tried to take on Russia.
The prospect of further UK sanctions against Russia comes a day after the prime minister announced that five Russian banks had had their assets frozen and three Russian billionaires would have travel bans imposed.
UK nationals were advised two weeks ago to leave Ukraine while commercial flights were still available. Those remaining have again been urged to leave Ukraine immediately – if they judge it is safe to do so.
Ukraine has closed its airspace to civilian flights. Wizz Air and Ryanair have suspended all flights to the country.