London's Metropolitan Police said in a post on X that people were arrested for showing support for a "proscribed organisation".
The gathering was the first since London's High Court ruled in February that a ban designating the pro-Palestinian group as a terrorist organisation was unlawful. UK Interior Minister Shabana Mahmood has been granted approval to appeal the ruling.
Lawmakers banned Palestine Action -- which accuses Britain's government of complicity in what it says are Israeli war crimes in Gaza -- under anti-terrorism legislation last July after some members broke into a Royal Air Force base.
Israel has repeatedly denied any abuses in its war in Gaza.
During Saturday's (local time) demonstration, placard-holding protesters, some wearing black and white Palestinian scarves and waving Palestinian flags, sat on the ground or on camping chairs.
