More than 100,000 protesters have joined an anti-immigration rally organized by a far-right activist on Saturday in central London.

The "Unite the Kingdom" rally, organized by far-right activist Stephen Yaxley-Lennon, known as Tommy Robinson, led to substantial police presence and violent clashes between protesters and police. This could be Britain's largest far-right rally in decades, according to local media.

London's Metropolitan Police appeared to be taken by surprise by the size of the turnout, describing the rally as "too big to fit into Whitehall" – a wide street lined with government buildings on the approved route of the march. Police trying to prevent protesters veering from the route faced "unacceptable violence," the force said, describing officers being kicked and punched and facing hurled bottles, flares and other projectiles. A total of 26 officers sustained injuries, four of them serious.

Arrests totaled 25, which the force said was "just the start." "We are identifying those who were involved in the disorder, and they can expect to face robust police action in the coming days and weeks," Assistant Commissioner Matt Twist said. The march brought a culmination to a highly charged summer in Britain that included protests staged outside hotels housing migrants. 

"We want our country back, we want our free speech back on track," said Sandra Mitchell, a supporter attending the rally. At the counter-protest, Ben Hetchin, a teacher, said, "The idea of hate is dividing us, and I think the more that we welcome people the stronger we are as a country." 

Police said they had deployed more than 1,600 officers across London on Saturday, including 500 brought in from other forces. 

The British government is facing mounting pressure over its tackling of irregular immigration. More than 1,000 migrants traveled across the English Channel on small boats last weekend, bringing the total number this year to over 30,000.

(With input from agencies)