UK Prime Minister Keir Starmer's office on Monday slammed US tech billionaire Elon Musk for using what it called "dangerous and inflammatory language" in a speech televised at one of the UK's largest-ever far-right protests.

Up to 150,000 people poured onto London streets on Saturday for the rally called by activist Tommy Robinson dubbed "Unite the Kingdom".

In his speech, Musk called for the dissolution of Britain's parliament and the replacement of Starmer's centre-left Labour government.

He also told the crowd that "violence is coming" and that "you either fight back or you die".

Starmer's spokesman said: "The UK is a fair, tolerant and decent country, and so the last thing the British people want is dangerous and inflammatory language, which threatens violence and intimidation on our streets."

The protesters marched over Westminster Bridge before rallying near Downing Street for speeches by figures associated with the far right from across Europe and North America.

"I think there's something beautiful about being British and what I see happening here is a destruction of Britain, initially a slow erosion but rapidly increasing erosion of Britain with massive uncontrolled migration," Musk said.

"A failure by the government to protect innocent people including children who are getting gang-raped. It's unreal the government has failed in its duty to protect its citizens which is a fundamental duty of government," he added.

He called for a change of government in Britain, saying: "You can't – we don't have another four years, or whenever the next election is, it's too long."

"Something's got to be done. There's got to be a dissolution of parliament and a new vote held."

Clashes erupted with police during the protest, with the Metropolitan Police saying 26 officers were injured

They estimated the attendance at "between 110,000 to 150,000, significantly exceeding the estimates of organisers".

Hope Not Hate, an anti-racism charity that has monitored such rallies for more than a decade, said "a number of well-known far-right extremists" including Robinson spoke on stage and were in the crowd.

Joe Mulhall, its director of research, told the BBC it was "probably... the largest far-right demonstration ever in Britain". (AFP)