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.

The government moved to ban Palestine Action under anti-terrorism laws last month after its activists broke into a Royal Air Force base and damaged two planes in protest against what the group said was Britain’s support for Israel.

Late on Friday, the campaign lost an urgent appeal against the parliamentary decision to proscribe it as a terrorist organisation, with the ban coming into force from midnight.

Under UK laws, offences include inviting support, expressing approval, or displaying symbols of a banned group and are punishable by up to 14 years in prison and/or a fine.

Britain has proscribed 81 groups under anti-terrorism laws, including Hamas, Al Qaeda and Daesh.

On Saturday, supporters gathered in Parliament Square in Westminster, some holding placards that said “I OPPOSE GENOCIDE. I SUPPORT PALESTINE ACTION.”

Sky News footage showed some being led away in handcuffs from a statue of Indian independence hero Mahatma Gandhi in the square, as they shouted their support.

United Nations experts have accused Israel of carrying out “genocidal acts” against Palestinians in the conflict in Gaza.

Israel has repeatedly dismissed such accusations.

Palestine Action has targeted Israel-linked companies in Britain in its protests, with interior minister Yvette Cooper saying that violence and criminal damage have no place in legitimate protest and that the group’s activities justify proscription.

Critics of the decision, including some United Nations experts and civil liberties groups, have argued that damaging property does not amount to terrorism.

At another protest on Saturday, the police arrested five pro-Palestine protesters from the Youth Demand group who threw red paint over a truck involved in London’s Pride parade and glued themselves to the vehicle. The parade has since resumed.