The UK government officially proscribed Palestine Action as a terrorist group as of midnight on Saturday.
It is now in the same category as al-Qaeda, ISIS, and Hezbollah. The UK's Metropolitan Police stated later on Saturday that membership or support for this group is a criminal offense, punishable by up to 14 years in prison.
British lawmakers voted on Wednesday to ban Palestine Action as a terrorist organization under the Terrorism Act 2000. The group has condemned the decision as an "abuse of power" and announced plans to challenge it in court.
The vote was delayed on Friday after Palestine Action's co-founder asked a London court to pause the British government's decision to ban the group under anti-terrorism laws, with her lawyers arguing the move was an "authoritarian abuse" of the law, Reuters reported.
British police arrested over 20 people on suspicion of terrorism offences after they showed support for the newly banned Palestine Action group in London, officials said on Saturday, hours after the proscription came into effect.
Scotland Yard warned ahead of the demonstration that those showing support for the group risked prosecution.
Leslie Tate, 76, a Green councillor from Hertfordshire who was at the protest, told the British newspaper: “Palestine Action are not a violent organisation, and the proscription is wrong.
“You do know, of course, that they were prescribed by Parliament with two other groups involved – all three at once so that was a trick to make sure the Bill went through.
“The evidence from their actions that they’ve taken from the start of Palestine Action is that they all have been non-violent.
“This protest is necessary to defend our democracy, and this is the creeping edge of totalitarianism, frankly.”
Yvette Cooper's move to ban Palestine Action
The UK moved to ban the pro-Palestine group in June. Home Secretary Yvette Cooper said that Palestine Action had created a “nationwide campaign of direct criminal action against businesses and institutions."
She added that the organization's “intimidatory and unacceptable” tactics were not representative of peaceful protest.
“The UK’s defense enterprise is vital to the nation’s national security, and this government will not tolerate those who put that security at risk,” Cooper added.
The group has targeted a number of institutions and private properties allegedly affiliated with Israel, including buildings housing a number of Jewish charities.
The group also claimed responsibility for the abduction and beheading of a statue of former Israeli President Chaim Weizmann from the University of Manchester.
More recently, the group has turned its attention to British military assets. Last month, it is understood that activists from Palestine Action broke into a military air base and destroyed two planes.
This is a developing story.
Reuters and Mathilda Heller contributed to this report.