Every person arrested on suspicion of supporting terrorist group Palestine Action in Westminster on Saturday has been released.
The total number handcuffed had risen to 522 after hundreds turned out to back the group, the Metropolitan Police said on Sunday.
That broke the record number of arrests for single protest event - thought to be 339 during the poll tax riots in 1990.
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One arrest took place at the form-up of the "Palestine Coalition" march in Russell Square, near King's Cross station, with the remaining 521 arrests at the protest in Parliament Square, the force said.
Hundreds turned out with placards reading: 'I support Palestine Action'
|PA
There were a further 10 arrests including six arrests for assaults on officers.
There was one arrest for obstructing a constable in the execution of their duty, two for breaching Section 14 Public Order Act conditions and one for a racially aggravated public order offence, the Met added.
Protest organisers Defend Our Juries released a statement on Sunday which read: "We can now confirm that every arrestee from the Lift The Ban sign-holding action has been released.
"None have been charged. Hundreds of the arrestees were taken to police cells.
"Hundreds were processed in pop-up gazebos after queuing for hours."
PALESTINE ACTION PROTESTS - LATEST:
The Home Secretary said that the right to protest was 'fiercely' protected
|PA
Later, the Met was forced to admit that policing the protest presented "entirely unrealistic" challenges.
Under the law, support for the group can lead to a prison sentence - but most of the weekend's protesters are expected to be fined.
As a result, officers were forced to bring in a "street bail" system typically used at mass eco-protests.
Police also set up "prisoner processing points" around Parliament Square to deal with some 320 protesters, who were asked to confirm their details before being let go and ordered to appear at a police station at a future date.
And though some were also handed conditions warning them not to attend Palestine Action protests, footage shared online appeared to show numerous protesters going on to return to Parliament Square.
Officers were forced to bring in a 'street bail' system typically used at mass eco-protests due to the sheer number of activists
|PA
Police later said it would have been impractical to try to re-arrest those who returned to the area.
One of those detained for holding signs reading "I oppose genocide... I support Palestine Action" was Sir Jonathon Porritt, a former Government adviser to Tony Blair and Gordon Brown.
He said it was a "privilege" to be arrested.
The Home Secretary said that the right to protest was "fiercely" protected, but warned that this was "very different from displaying support for this one specific and narrow proscribed organisation".
And while No10 says it expects charges to be brought against anyone who has broken the law, the final decision will rest with Attorney General Lord Hermer.