The 74-year-old woman was arrested in Belfast on Saturday, while others received cautions for wearing T-shirts with the slogan “We are all Palestine Action”.

Palestine Action, a non-violent direct action group, has been designated by the UK government as a proscribed organisation under the Terrorism Act 2000.

MPs approved the designation after members of the group damaged two British military planes at RAF Brize Norton by spraying them with red paint.

However, in a letter sent to the PSNI’s chief constable, Jon Boutcher, Amnesty International says the arrest of peaceful protesters solely for expressing support for Palestine Action is a violation of the UK’s international obligations to protect the rights of freedom of expression and peaceful assembly.

Patrick Corrigan, Amnesty’s Northern Ireland director, said: “The PSNI must uphold people’s right to protest against the genocide being perpetrated in Gaza. Arresting people on terrorism offences for wearing a T-shirt is the height of absurdity.”

In the letter, Mr Corrigan details how the arrest of otherwise peaceful protesters solely for expressing a statement such as ‘I Oppose Genocide. I Support Palestine Action’ is a violation of the UK’s international obligations to protect the rights of freedom of expression and assembly.

“Under international human rights law, protest speech should only be criminalised if it incites violence, hatred or discrimination,” he writes.

“In the case of the Palestine Action solidarity protests, holding a placard or wearing a T-shirt and peacefully stating support for Palestine Action cannot be treated as an example of incitement.”

The letter goes on to say that arrests on this basis would violate international human rights law. As such, Amnesty is urging the PSNI chief constable to instruct officers to comply with the UK’s international obligations and act with discretion in their response to peaceful protesters.

The letter also argues that given the High Court in London has ruled that it is at least arguable that the proscription of Palestine Action was unlawful, the legal foundation for the offences under which arrests have taken place so far is also in doubt.