Merseyside Police said more than 50 people had so far been detained at the Defend Our Juries event and that officers were in the process of making arrests on suspicion of wearing/carrying an article supporting a proscribed organisation.
A force spokesperson said it respected everyone’s rights to protest lawfully while also aiming to keep the peace and ensure public safety.
The force said it made “detailed and co-ordinated plans ahead of such events to ensure it can apply this consistently, fairly and proportionately”.
The spokesperson said not everyone present at the protest had been committing a criminal offence.
The arrests were criticised by Amnesty International, which called them a “shameful attack on freedom of speech”.
One of those detained was human rights barrister Margaret Owen, 94.
A spokesperson for Defend Our Juries said: “We had hoped the new Prime Minister, Andy Burnham, would do the sensible thing and bin the ban on Palestine Action given the weight of public pressure to do so and the escalating amount of resources required in enforcing it.
“Continuing the charade of the proscription of Palestine Action is illogical, unsustainable and immoral.”