The Metropolitan Police said it had made the arrests, thought to be one of the highest number ever at a single protest in the UK capital, for "supporting a proscribed organisation".
It also arrested eight people for other offences including five for alleged assaults on officers, though none were seriously injured, it added.
The government outlawed Palestine Action in early July, days after it took responsibility for a break-in at an air force base in southern England that caused an estimated �7 million ($9.3 million) of damage to two aircraft.
The group said its activists were responding to Britain's indirect military support for Israel amid the war in Gaza.
Britain's interior ministry reiterated ahead of Saturday's protests that Palestine Action was also suspected of other "serious attacks" that involved "violence, significant injuries and extensive criminal damage".
But critics, including the United Nations and groups such as Amnesty International and Greenpeace, have condemned 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. For him, the ban was "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, who nearly all appeared to offer no resistance, attendees 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."
However, interior minister Yvette Cooper insisted late Saturday Palestine Action had been outlawed "based on strong security advice" and following "an assessment from the Joint Terrorism Assessment Centre that the group prepares for terrorism".
"Many people may not yet know the reality of this organisation," she said, adding it "is not non-violent".
"The right to protest is one we protect fiercely but this is very different from displaying support for this one specific and narrow, proscribed organisation."
- NGOs opposed -
Police forces across the UK have made scores of similar arrests since the government outlawed Palestine Action on July 5, making being a member or supporting the group a criminal offence punishable by up to 14 years in prison.
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.
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 arrest of otherwise peaceful protesters is a violation of the UK's international obligations to protect the rights of freedom of expression and assembly," Amnesty said Saturday on X.
A UK court challenge against the decision to proscribe Palestine Action will be heard in November.
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