Similar demonstrations are being held in Manchester, Edinburgh, Bristol and Truro after the group was officially designated as a proscribed terrorist organisation two weeks ago.
Protesters gathered in Parliament Square carrying white placards with the message: “I oppose genocide, I support Palestine Action.”
Some were carried away by officers, while others were led away in handcuffs.
The Metropolitan Police said in a post on X: “55 people were arrested in Parliament Square for displaying placards in support of Palestine Action which is a proscribed group.”
The arrests were made under Section 13 of the Terrorism Act 2000, which carries a maximum six-month prison sentence.
Seventy people were arrested at similar demonstrations in Parliament Square over the past two weekends.
Eight people were arrested near Truro Cathedral under Section 13, Devon and Cornwall Police said, while protesters have also been arrested at a demonstration in Manchester.
Before Saturday’s protests, Defend Our Juries, which is coordinating the campaign, said a total of 120 people had previously been arrested across the UK.
The Met warned anyone expressing support for Palestine Action would likely face arrest and that officers would be on the lookout for chants similar to “death to the IDF” led by punk rap duo Bob Vylan at Glastonbury Festival.
Home Secretary Yvette Cooper announced plans to proscribe Palestine Action after two Voyager aircraft were allegedly damaged at RAF Brize Norton in Oxfordshire on 20 June, which police said caused around £7m worth of damage.
The ban means that membership of, or support for, the direct action group is now a criminal offence punishable by up to 14 years in prison, under the Terrorism Act 2000.
Even wearing a T-shirt or badge with the group’s name on attracts a maximum six-month sentence.
Huda Ammori, the co-founder of Palestine Action, is seeking to challenge the ban, with a hearing for permission to bring a judicial review set to take place at the High Court on Monday.
