The award-winning Scottish screenwriter Paul Laverty has been arrested in Edinburgh for wearing a T-shirt that allegedly referenced the proscribed protest group Palestine Action.

Laverty was attending a protest outside St Leonard’s police station in the city centre to support Moira McFarlane, a member of Scottish Palestine Solidarity Campaign, who was due to be charged under section 13 of the Terrorist Act for wearing a T-shirt with the words: “Genocide in Palestine, time to take Action”.

Police Scotland confirmed that a 68-year-old man had been arrested under the Terrorism Act for “showing support for a proscribed organisation” and inquires were ongoing.

Laverty, a long-term collaborator of the film-maker and activist Ken Loach, has written screenplays for films including Carla’s Song, Sweet Sixteen and I, Daniel Blake. Their collaboration on The Wind That Shakes the Barley, about the Irish war of independence, won the Palme d’Or at Cannes.

There have been hundreds of individual and mass arrests of people attending rallies referencing Palestine Action across the UK since the ban on the group came into force on 5 July, after a high court judge refused to grant the group’s co-founder Huda Ammori an injunction suspending it while a judicial review was pending.

Yvette Cooper, the home secretary, announced the ban days after activists from the group broke into RAF Brize Norton and defaced two military aircraft with spray paint.

Police in London are braced for further arrests after Defend Our Juries, the pressure group leading protests against the ban, called for more than 1,000 people to sign up for its next demonstration to be held in London on 6 September.

Earlier this month, 532 people were arrested at the largest demonstration so far relating to Palestine Action since it was proscribed.

Last week, the former Labour cabinet minister Peter Hain said the UK government was “digging itself into a hole” over Palestine Action and that fellow Labour peers and MPs were regretting voting to ban the group.

In an article for the Guardian, co-written with Labour MP Stella Creasy, the pair wrote: “The status quo has come to mean equating peaceful witness with terrorism, and isn’t sustainable. But neither is pretending there isn’t a problem that threatens our ability to debate, disagree and ultimately decide in our democracy.”