Counter terror police have arrested five activists linked to Palestine Action in a series of dawn raids.
Three men and two women, aged between 26 and 61, were taken into custody on Tuesday as part of a “proactive investigation” led by the Metropolitan Police’s Counter Terrorism Command, the force said.
Campaign group Defend Our Juries said more than 1,000 people are set to take action on September 6 in Parliament Square, holding signs saying "I oppose genocide. I support Palestine Action".
This follows several demonstrations over the summer that have seen more than 700 people arrested and 114 charged for supporting the proscribed group.
Supporters including Tim Crosland and law student Paddy Friend were arrested by counter-terrorism police, Defend Our Juries announced.
Mr Friend had reportedly hosted a Zoom call with other members to give legal advice to those planning to take part in the mass protest action.
A spokesperson for Defend Our Juries said: "This is scandalous. Locking up our key spokespeople just hours before they were due to hold a press conference announcing more peaceful Lift the Ban protests constitutes an unprecedented assault on free speech in our country.
"This level of political repression is not what we expect in a democracy - it's the kind of tactic typically associated with authoritarian regimes around the world.
"Despite this dystopian crackdown targeting perceived organisers of peaceful protests, the mass action will go ahead, with 1,000 people pledging to hold signs.
"The mass defiance of the unjust ban on Palestine Action is unstoppable.”
The five were arrested under section 12 of the Terrorism Act, which covers support for banned groups.
The Met confirmed officers arrested a 26-year-old man and a 55-year-old man in South London, a 61-year-old woman in East London, a 48-year-old woman in Kendal, Cumbria and a 39-year-old man in Barnsley, South Yorkshire on Tuesday morning.
Chief Superintendent Helen Flanagan, Head of Operations for the Counter Terrorism Command said: “These arrests have been made as part of an ongoing investigation into people we suspect have been involved in encouraging support for the proscribed group Palestine Action.
“We know there is a great strength of feeling towards the situation in Palestine and thousands have been able and continue to be able to express their views through protest and demonstrations, without breaking counter terrorism laws.
“But Palestine Action is clearly proscribed as a terrorist group, and those showing support for this particular group, or encouraging others to do so can expect to be arrested, investigated and prosecuted.
“There are serious consequences for those who are found guilty of an offence under the Terrorism Act, so I would urge anyone considering showing public support towards to this proscribed group to reconsider.” Those arrested have been taken into police custody, where they currently remain and enquiries remain ongoing.”
Kerry Moscogiuri, director of campaigns and communications at Amnesty International UK, branded the arrests "incredibly concerning" and said those held should be immediately released.
"The mass peaceful demonstrations they have organised over recent weeks are protected by international human rights law - and to think they run the risk of being imprisoned for up to 14 years is a terrifying example of the UK authorities' willingness to use authoritarian practices to silence dissent," she added.
