The UK capital's Metropolitan Police said it expected to make further arrests at the demonstration in Parliament Square, as organisers claimed only a "fraction" of the hundreds who turned out had been detained.

"That claim simply isn't true," the Met said in a statement, noting some of those there were onlookers or not visibly supporting Palestine Action.

"We are confident that anyone who came to Parliament Square today to hold a placard expressing support for Palestine Action was either arrested or is in the process of being arrested."

The government banned the group days after several of its activists broke into an air force base in southern England, causing an estimated pound7 million ($9.3 million) of damage to two aircraft.

Britain's interior ministry reiterated ahead of Saturday's protests that its members were also suspected of other "serious attacks" that involved "violence, significant injuries and extensive criminal damage".

But critics, including the United Nations and NGOs like Amnesty International and Greenpeace, have lambasted the move as legal overreach and a threat to free speech.

- 'Unprecedented' -

A group called Defend our Juries, which organised Saturday's protests and previous demonstrations against the ban, said "unprecedented numbers" had risked "arrest and possible imprisonment" to "defend this country's ancient liberties".

"We will keep going. Our numbers are already growing for the next wave of action in September," it added.

Attendees began massing near parliament at lunchtime bearing signs saying "oppose genocide, support Palestine Action" and other slogans, and waving Palestinian flags.

Psychotherapist Craig Bell, 39, was among those holding a placard.

He branded the ban "absolutely ridiculous".

"When you compare Palestine Action with an actual terrorist group who are killing civilians and taking lives, it's just a joke that they're being prescribed a terrorist group," he told AFP.

As police moved in on the demonstrators, they applauded those being arrested and shouted "shame on you" at officers.

"Let them arrest us all," said Richard Bull, 42, a wheelchair-user in attendance.

"This government has gone too far. I have nothing to feel ashamed of."

- NGOs opposed -

London's Met Police and other UK forces have made scores of similar arrests on previous weekends since the government outlawed Palestine Action on July 5.

Anyone expressing support for a proscribed group risks arrest under UK anti-terror laws.

Police announced this week that the first three people had been charged in the English and Welsh criminal justice system with supporting Palestine Action following their arrests at a July 5 demo.

Being a member or supporting the group is now a criminal offence punishable by up to 14 years in prison.

Seven people have so far been charged in Scotland, which has a separate legal system.

Amnesty International UK Chief Executive Sacha Deshmukh wrote to Met Police chief Mark Rowley this week urging restraint be exercised when policing people holding placards expressing support for Palestine Action.

The NGO has argued arrests of such people are in breach of international human rights law.

A UK court challenge against the decision to proscribe Palestine Action will be heard later this year.