Clashes erupted on Saturday as more than 100,000 people turned out in central London for a rally organised by far-right activist Tommy Robinson, with police arresting nine people after facing "unacceptable violence" trying to control the crowds.

The huge crowds, many draped in English and British flags, gathered from late morning in and around Westminster for what Robinson, a veteran of UK far-right organising, branded the country's "biggest free speech festival".

Meanwhile around 5,000 people turned out at a "Stand Up to Racism" march starting around a mile to the north, with London's Metropolitan police deploying about 1,000 police to keep the rival groups apart.

The Met, which estimated 110,000 people attended the far-right rally, said the clashes erupted after some were unable to access the main stage area and tried to enter so-called "sterile areas" near counter-demonstrators.

"When officers moved in to stop them they faced unacceptable violence," the force said.

"They were assaulted with kicks and punches. Bottles, flares and other projectiles were thrown."

"Nine arrests have been made so far for various offences, but many more people have been identified as committing offences," it added, vowing to find them "even if it is not possible to do so today".

Robinson's latest "Unite the Kingdom" event saw attendees march over Westminster Bridge before rallying near Downing Street for speeches by far-right figures from across Europe and North America.

"The silent majority will be silent no longer," Robinson told the crowd. "Today is the spark of a cultural revolution." (AFP)