Russia is facing international condemnation after launching an attack on Ukraine.
Russian President Vladimir Putin warned other countries that any attempt to interfere would lead to "consequences you have never seen".
Big explosions were heard before dawn in Kyiv, Kharkiv and Odesa as world leaders decried the start of an invasion that could cause massive casualties, topple Ukraine's democratically elected government and threaten the post-Cold War balance on the continent.
Watch: PM - Putin's 'Hideous and barbarous venture' in Ukraine must end in failure
Ukrainians started fleeing some cities, and the Russian military claimed to have incapacitated all of Ukraine's air defences and airbases within hours.
Ukrainian President Volodymyr Zelensky declared martial law, saying Russia has targeted Ukraine's military infrastructure.
Boris Johnson said Mr Putin had "chosen a path of bloodshed and destruction" in an "unprovoked attack", and updated the Commons on the UK's response to the crisis - including what he described as the "largest and most severe" package of economic sanctions Russia had ever seen.
Mr Johnson said "at this stage, Ukrainians are offering a fierce defence of their families and their country".
"Earlier today, President Putin delivered another televised address and offered the absurd pretext that he sought the 'demilitarisation and denazification of Ukraine'.
"In fact, he is hurling the might of his military machine against a free and peaceful neighbour.
"For this, Putin will stand condemned in the eyes of the world and of history.
Watch: Inside the mind of Vladimir Putin.
"He will never be able to cleanse the blood of Ukraine from his hands."
The Prime Minister added he was "driven to conclude that Putin was always determined to attack his neighbour" and "now we see him for what he is, a bloodstained aggressor who believes in imperial conquest".
"Now, we have a clear mission – diplomatically, politically, economically and, eventually, militarily," he said.
"This hideous and barbarous venture of Vladimir Putin must end in failure."
It comes after a Downing Street spokesman said Mr Johnson assured Mr Zelensky the West will "not stand by as President Putin waged his campaign against the Ukrainian people".
"The Prime Minister said he hoped Ukraine could resist, and that Ukraine and its people were in the thoughts of everyone in the United Kingdom people during this dark time," the spokesman said.
Watch: Inside the mind of Vladimir Putin
In a tweet on Thursday evening, the Ministry of Defence (MOD) said there were "continued reports of clashes along the Line of Contact in the Donbas, with no breakthrough by Russian Forces".
They added the Russian Black Sea Fleet had entered Ukrainian waters in the Black Sea and Sea of Azov, with civilian shipping suspended in the latter "in preparation for a full blockade".
However, the post did say they had seen "no evidence" of an amphibious landing in Odesa.
US President Joe Biden pledged new sanctions to punish Russia for the aggression that the international community had expected for weeks but could not prevent through diplomacy.
Mr Putin announced the action during a televised address, claiming the attack was needed to protect civilians in eastern Ukraine – a claim the US had predicted he would make as a pretext for an invasion.
He accused the US and its allies of ignoring Russia's demands to prevent Ukraine from joining NATO and for security guarantees.
He also claimed that Russia does not intend to occupy Ukraine but will move to "demilitarise" it.
Mr Biden in a written statement condemned the "unprovoked and unjustified attack", and he promised that the US and its allies would "hold Russia accountable".