By Michael Holden and Will Russell

LONDON (Reuters) -More than 300,000 pro-Palestinian demonstrators marched through central London on Saturday, with police arresting over 120 people as they sought to stop far-right counter-protesters from ambushing the main rally.

Skirmishes broke out between police and the far-right groups gathered to protest against the demonstration taking place on Armistice Day, the anniversary of the end of World War One, when Britain commemorates its war dead.

Prime Minister Rishi Sunak condemned the violence seen at the Cenotaph war memorial and also attacked "Hamas sympathisers" who joined the bigger rally, "singing antisemitic chants and brandishing pro-Hamas signs and clothing on today's protest".

Tensions had been running high before Saturday's march - the biggest in a series to show support for the Palestinians and call for a ceasefire in the Gaza Strip - after interior minister Suella Braverman called them "hate marches" led by "mobs".

London's Metropolitan Police had refused ministerial requests to block the event, saying they did not have indications that there would be serious violence, straining relations with the government.

Police said in a statement late on Saturday that they had arrested 126 people so far, the majority of whom were right wing protesters who formed part of a group several hundred strong which police said included football hooligans.

"The extreme violence from the right wing protesters towards the police today was extraordinary and deeply concerning," Assistant Commissioner Matt Twist said, adding that a knife and knuckleduster were found during searches.

The intense debate about protest and policing in the run-up to the march had raised community tensions, he said.

While the much larger pro-Palestinian rally did not see physical violence, the senior officer said small groups had broken away from the main march, and about 150 people wearing face coverings had fired fireworks which struck officers in their faces, leading to arrests.

Investigations into a small number of hate crime and support for proscribed organisation offences were also ongoing, he said.

Sunak called for the police to take a tough line.

"All criminality must be met with the full and swift force of the law," he said in a statement late on Saturday. "That is what I told the Met Police Commissioner on Wednesday, that is what they are accountable for and that is what I expect."

Skirmishes between police and right wing protesters continued throughout the day, with police in riot gear using batons to try to contain protesters who threw bottles.

London Mayor Sadiq Khan and Scotland's first minister, Humza Yousaf, said Braverman had inflamed tensions and emboldened the far-right by accusing the police of favouring "pro-Palestinian mobs" before the event.

OVER 300,000 JOIN MARCH

Police said more than 300,000 had joined the pro-Palestinian rally, while organisers put the figure at 800,000.

Some marchers chanted "From the river to the sea, Palestine will be free", a rallying cry viewed by many Jews as antisemitic and a call for Israel's eradication.

Others carried banners reading "Free Palestine", "Stop the Massacre" and "Stop Bombing Gaza".

Since Hamas's assault in southern Israel on Oct. 7, there has been strong support and sympathy for Israel from Western governments, including Britain's, and many citizens. But the Israeli military response has also prompted anger, with weekly protests in London demanding a ceasefire.

In the U.S., hundreds of protesters rallied near President Joe Biden’s house in Wilmington, Delaware, shortly before he arrived home for the weekend. Biden’s motorcade circumvented the demonstration, in which hundreds also walked down the tree-lined street to get closer to the Biden residence. Many wore the Palestinian flag and carried signs demanding a ceasefire.

Hanaa A., who declined to provide her last name, said she came to Wilmington to protest the killing of innocent children and demand an immediate ceasefire. Asked about the likely contest next November between Biden and former President Donald Trump she said "We will pray and wait, but at this point, we will not elect him (Biden).”

About 21,000 people took part in a pro-Palestinian rally in Brussels on Saturday, and in Paris, left-wing lawmakers were among some 16,000 protesters who marched with pro-Palestinian banners and flags to call for a ceasefire.

Some French leftist politicians have welcomed President Emmanuel Macron's call this week for a ceasefire and opposition to Israel's bombing campaign.

Senior French lawmakers have called a protest against antisemitism for Sunday.

(Reporting by Michael Holden, Hannah McKay, Hollie Adams, Ben Makori, Will Russell, Natalie Thomas, Alishia Abodunde, Yann Tessier, Gerhard Mey and Dylan Martinez in London, Lucien Libert, Claudia Greco in Paris, Bart Biesemans in Brussels and Andrea Shalal in Wilimington, Delaware; Writing by Sarah Young; Editing by Kate Holton, Ed Osmond, Helen Popper, Kevin Liffey, Daniel Wallis and David Gregorio)