Britain on Thursday imposed a biting package of sanctions on Russia that Prime Minister Boris Johnson said would degrade its economy "for years to come", as he slammed President Vladimir Putin in unusually personal terms.

After Putin ordered his forces to invade Ukraine, Johnson called him a "dictator" who "will never be able to cleanse the blood of Ukraine from his hands".

Downing Street will fly Ukraine's flag and be lit up in the national colours of yellow and blue on Thursday evening, the prime minister's spokesman said.

Protesters waving Ukrainian flags and brandishing placards of support gathered outside Downing Street.

The UK sanctions include freezing the assets of Russian bank VTB and arms manufacturer Rostec, sanctions on five more oligarchs close to Putin including his former son-in-law Kirill Shamalov, and banning Aeroflot from British airspace.

Britain will legislate to prevent the Russian state and entities from raising money in London, and ban the export of "dual-use" equipment that can have military applications.

It will also legislate to limit how much money Russians can hold in UK bank accounts, although the ceiling has yet to be determined.

Long accused of turning a blind eye to Kremlin-backed money flowing through London, the government will accelerate an "Economic Crime Bill", notably to prise open the real ownership of Russian-held assets.

Johnson told parliament the measures were "the largest and most severe package of economic sanctions that Russia has ever seen".

'Blood-Stained Aggressor'

Putin would "stand condemned in the eyes of the world and of history", he added.

"Putin was always determined to attack his neighbour, no matter what we did," he said, calling the Kremlin chief "a blood-stained aggressor who believes in imperial conquest".

Britain had this week already imposed sanctions on five Russian banks and three billionaire businessmen, but was accused of not going far enough.

The new trade sanctions, Johnson said, "will constrain Russia's military industrial and technological capabilities for years to come".

"We cannot and will not just look away," he had said in a televised address to the nation, after phoning Ukraine's President Volodymyr Zelensky just after four am (0400 GMT) as Russian forces moved in.

Johnson summoned his security chiefs for an early meeting in response to the invasion and was due to hold a further crisis session and full cabinet meeting on Thursday evening.

Foreign Secretary Liz Truss said the government had deployed teams to five countries in eastern Europe to support Britons leaving Ukraine.

Transport Secretary Grant Shapps said he had instructed the UK Civil Aviation Authority to ensure airlines avoid Ukraine airspace "to keep passengers and crew safe".

The other Russians sanctioned Thursday by the UK were Pyotr Fradkov, Denis Bortnikov, Yuri Slyusar and Elena Georgieva.

The oligarchs are "people who have international lifestyles", a British diplomatic source said.

"They come to Harrods to shop, they stay in our best hotels when they like, they send their children to our best public (private) schools, and that is what's being stopped.

"So these people are essentially persona non grata in every major Western European capital in the world. That really bites."

Johnson said sanctions would also be extended to Belarus for helping in the invasion, but did not go into detail.

Russia, meanwhile, is due to hold the final of European football's Champions League final in Saint Petersburg on May 28, which governing body UEFA is reviewing.

"I cannot for the life of me see how that can currently go ahead," Johnson told parliament.