A large pro-Palestinian march kicked off in London on Saturday following scuffles nearby between far-right protesters and police, who launched a major operation to avert clashes between the two rival groups.
The pro-Palestinian march, which is expected to attract hundreds of thousands of people, has also drawn counter-protesters from right-wing groups to the capital on what is Britain's day of remembrance for war veterans.
London's Metropolitan Police said in a post on X, formerly Twitter, they had faced aggression from counter-protesters who were in the city in "significant numbers", adding that they would not allow them to confront the pro-Palestinian rally.
"We will use all the powers and tactics available to us to prevent that from happening," the police said.
The "National March for Palestine" is the latest in a series of rallies in the British capital to show support for the Palestinians and call for a ceasefire from Israel's bombardment of the Gaza Strip.
Government ministers had called for Saturday's march to be cancelled because it falls on Armistice Day, which marks the end of World War One and commemorates those killed in military action.
Ben Jamal, one of the organisers from the Palestine Solidarity Campaign (PSC), told Reuters up to a million people could join the rally. He said it would be peaceful, but acknowledged the "heightened situation today".
As they gathered at the start point, pro-Palestinian protesters could be heard shouting "From the river to the sea, Palestine will be free", a rallying cry which is viewed by many Jews as antisemitic and a call for Israel's eradication.
The scuffles between police and the far-right counter-protesters took place close to the Cenotaph war memorial earlier on Saturday, where some of the counter-protesters chanted: "We want our country back."
Bottles were later thrown at police by members of right-wing groups in a separate incident in Chinatown, about a mile north of the war memorial, police said.
Police have said almost 2,000 officers will be on duty to prevent disorder and an unprecedented 24-hour police guard at the Cenotaph has been in place since Thursday.
Deputy Assistant Commissioner Laurence Taylor said on Friday that Saturday would be "challenging and tense".
Prime Minister Rishi Sunak has criticised the pro-Palestinian rally on Armistice Day as disrespectful.
While previous PSC marches have been generally peaceful, there have been more than 100 arrests for offences including showing support for Hamas, which is banned as a terrorist organisation in Britain, or holding placards with offensive slogans.
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.
Home Secretary Suella Braverman, the minister in charge of policing, has courted controversy by calling the protesters "hate marchers", and Sunak has come under pressure from his own lawmakers to sack her after she accused the police of double standards over how they treated "pro-Palestinian mobs".
Scottish First Minister Humza Yousaf on Saturday called for her to resign, blaming her for emboldening the far-right protesters.
"She has spent her week fanning the flames of division," he said on X.