Police said 25 people were arrested under the Terrorism Act at a protest in Birmingham where “a number of people carried placards expressing support for Palestine Action”.
The arrests were made at a demonstration on Saturday organised by Defend Our Juries at Chamberlain Square in the city centre, West Midlands Police said.
Those arrested were taken into custody under Section 13 of the Terrorism Act 2000.
West Midlands Police said: “We arrested 25 people at a protest in Birmingham city centre this afternoon.
“The arrests were made during a demonstration organised by Defend Our Juries in Chamberlain Square, at which a number of people carried placards expressing support for Palestine Action.
“The group was proscribed by the Government in July this year, meaning it is an offence to be a member of it or express support for it.”
Chief Inspector James Littlehales said: “We fully recognise people’s right to demonstrate peacefully, and many people continue to protest in support of Palestine without breaking the law.
“But we will take appropriate action where people are breaching the law by showing support for proscribed organisations.”
Defend Our Juries organised demonstrations in 10 British towns and cities on Saturday afternoon, to protest “against our Government’s complicity in genocide and against the ban on Palestine Action”.
Ten people were also arrested for suspected terror offences in Norwich on Saturday after allegedly holding Palestine Action placards at a protest.
After being arrested on suspicion of displaying an item in support of a proscribed organisation, they are being held at Wymondham Police Investigation Centre for questioning, officers said.
At 12.30pm on Saturday, Norfolk Constabulary attended a group who assembled on Haymarket in the city centre. Officers said the “incident was resolved” at around 3pm.
Superintendent Terry Lordan said: “As we have seen throughout the summer, officers will always work to protect the democratic right to assembly and facilitate peaceful protest.
“Our role as a police force is to prevent disorder, damage and disruption in the local community. Today, this involved arresting individuals who were committing offences under the Terrorism Act.
“The actions of this group were unlawful and officers have used their powers accordingly.”
A separate march in London, organised by the Palestine Coalition, drew an estimated 100,000 people, according to organisers.
The Metropolitan Police said one arrest was made during the protest for affray.
Four people were “detained on suspicion of Public Order Act offences and inquiries are ongoing”, police added.
The demonstrators, many of whom were carrying Palestine flags, chanted “From the sea to the river, Palestine will live forever” and “Israel is a terrorist state” as they processed from Green Park to Whitehall.
Some of the demonstrators set off green and red fireworks as they walked down Piccadilly in central London.
Labour MP Bell Ribeiro-Addy told demonstrators in London that “the UK must stop enabling violence”.
Addressing protesters in Whitehall, Ms Ribeiro-Addy said: “We’re here because we know that even as political leaders congratulate themselves on this so-called ceasefire, the genocide in Gaza continues in real time.”
Ms Ribeiro-Addy added: “A ceasefire in name only is not a ceasefire, it’s a political risk management exercise. It’s designed to protect foreign partners and deflect domestic accountability.
“What’s happening on the ground is completely incompatible with the language being used in press and parliamentary briefings.
“The UK’s role hasn’t changed either, you cannot credibly call for peace abroad while enabling violence through policy at home.”
Poplar and Limehouse MP Apsana Begum said demonstrators would not stop marching until there was a “free and independent Palestine”.
