Police arrested an activist on suspicion of supporting a banned organisation as thousands of pro-Palestinian protesters prepared to march near to the Israeli embassy on Saturday.

Scotland Yard said the arrest had been made “on suspicion of support for a proscribed organisation in relation to a placard”.

Between 200,000 and 250,000 people were expected to gather for the demonstration, according to a spokesman for the Palestine Solidarity Campaign, which kicked off at 1.30pm near Marble Arch.

Many of the protesters held placards proclaiming: “From the river to the sea, Palestine will be free”, which is widely seen as a call for the destruction of the state of Israel.

Others brandished banners accusing Israel of genocide.

There were also claims that some of the marchers held placards with symbols showing support for Hamas gunmen, such as one appearing to show a V on a red background in imitation of the terror group’s red triangle symbol.

Around 1,500 police officers from forces across the UK are set to be on public order duties during the demonstration.

Activists were massing close to the Israeli embassy in Kensington for the second time since the October 7 attacks launched on southern Israel by Hamas.

The Met Police intervened to prevent the march starting until after a religious service at a synagogue along the route had concluded – although hundreds of pro-Palestine activists gathered nearby before the demonstration set off.

The Campaign Against Antisemitism (CAS) described the choice of route past the synagogue as deliberately intimidating.

The CAS said: “In previous weeks, the marches have included people supporting Hamas and openly flaunting their anti-Jewish racism, and congregants leaving synagogue had to walk through them.”

The Met admitted many would question why the protest so close to the embassy was allowed, but said it was a “common misconception” that forces can allow or refuse permission for a protest to take place.

Scotland Yard added that a “real risk of serious disorder” sufficient for the Met to request that the Home Secretary ban the protest had not been seen at recent demonstrations and was not expected on Saturday.

Protesters were being kept more than 100 metres away from the embassy grounds and face arrest if they move closer. A static rally was held near the embassy on October 9.

Conditions under Section 12 of the Public Order Act prevent any person participating in the march from deviating from the route and police have also said no gazebos or other stalls could be erected in a specified area at Marble Arch.

All officers have been “briefed to be on the lookout for offensive placards and banners” and police teams were monitoring CCTV of the demonstration for evidence of hate speech and anti-Semitic banners and placards.

Commander Kyle Gordon, who was leading the policing operation, said: “We are there to ensure protests take place lawfully, minimising disruption to the life of the wider public and in a way that gives due consideration to the cumulative impact on London’s communities and those who feel most vulnerable in the current climate.

“The protests we have seen since October have thankfully been largely peaceful and we must take this into account in our policing approach. I hope the same will be true this Saturday.

“Unfortunately, despite this, we have regularly seen officers having to deal with offences related to placards and other hate speech.

“We do not underestimate the fear this causes, nor the impact of such criminal and unacceptable behaviour on wider community relations.”

At least 28,663 Palestinians have been killed in Gaza since Israel began its military operation in response to the October 7 attacks, when militants killed some 1,200 people and took another 250 hostage.

Ben Jamal, director of the Palestine Solidarity Campaign (PSC), said: “The images this week from Israel’s bombardment of Rafah, of children with limbs torn apart, should be seared on the conscience of the world.

“Despite mounting pressure from world leaders, and in defiance of the ICJ ruling, the Israeli government has made clear that it is about to launch an attack on Rafah that will lead to unprecedented levels of carnage.

“The moral imperative is clear. An immediate ceasefire is a simple, absolute necessity.

“The legal imperative is also clear, the UK must abide by its responsibilities under the Genocide Convention to cease any activities that make it complicit.”