Police were “surrounded” by demonstrators and prevented from leaving after making two arrests at a pro-Palestinian march in south London.
The arrests came as Liz Truss, the former prime minister, claimed on Fox News that people are being allowed to “demonstrate in favour of terrorists”.
The Metropolitan Police said it had arrested a man for a racially aggravated public order offence in Brixton, and another, a teenager, on suspicion of criminal damage.
The protest in Brixton was one of dozens taking place across the country on Saturday in a day of action organised by the Palestine Solidarity Campaign (PSC).
The group is demanding an immediate and permanent ceasefire in the Israel-Hamas war after fighting resumed on Friday and said its primary motivation was concern over further civilian casualties.
The Met declined to provide details about the content of the placard it deemed to be potentially “racially aggravating”, but numerous posters on display during weekly pro-Palestinian marches have been described as anti-Semitic by campaign groups.
The force said protesters obstructed their route as they attempted to leave the Windrush Square, a pedestrianised area in the centre of Brixton, having arrested the first man.
In a statement on X, formerly Twitter, the force said: “When officers tried to leave the area, their vehicle was surrounded by other protesters. Some sat in the road blocking their path. Additional officers were deployed and the arrested man is now on his way to custody.”
Turning onto Saltoun Road, the police van was again stopped by protesters and a male teenager was arrested for criminal damage to the vehicle. A spokesman for the Met said that “a number of people tried to obstruct the police van as it drove off. They were moved out of the way by officers”.
The force had been criticised for being “too soft” at initial marches in the capital. However, last weekend it warned demonstrators that it would take incitement to violence or racist language far more seriously.
Officers handed out leaflets to provide “absolute clarity” on what was deemed an offence, while Arabic-speaking officers were deployed to pick out offensive chants and images in the crowd.
In response to the resumption of hostilities in Gaza, protesters gathered at rallies and vigils across the UK and Ireland on Saturday. While the marches were largely peaceful, protesters across the country continued to chant slogans condemned by many as anti-Semitic.
On the streets of Dublin, some marchers called for an “intifada”, while at Plashet Park in East Ham, east London, there were chants accusing Israel of being a “terrorist state”.
Ms Truss, who has been on a trip to the United States as part of a delegation with the Conservative Friends of Ukraine, claimed the protests showed the “woke Left” would “rather support authoritarian regimes” than the West.
She said: “On the streets of London, I see people demonstrating in favour of terrorists, and that being allowed to happen.
“They are about saying: ‘The way of life that we have in Britain or America, that is not the right way of life. Actually, we would rather support terrorists, we’d rather support authoritarian regimes.’”
Jeremy Corbyn, the former Labour leader who has previously refused to condemn Hamas as terrorists, appeared as the keynote speaker at a Stop the War Coalition event in east London.
Journalists attending the “Freedom for Palestine; Imperialism, War and the Middle East” were told the meeting at a small conference centre was not open to them.