The demonstration against the banning of Palestine Action as a terror group has been "the picture of peaceful protest", organiser Defend Our Juries (DOJ) has claimed after the Metropolitan Police said its officers have been "subjected to an exceptional level of abuse".
The force said officers have been punched, kicked, spat at and had objects thrown at them as they policed the Westminster rally in which an estimated 1,500 people took part - many holding signs that read: "I support Palestine Action".
Violent clashes broke out as tensions rose through the afternoon - with the protest taking on an increasingly anti-police tone featuring chants of "shame on you", "you're supporting genocide" and some referencing former officer and murderer Wayne Couzens.
The Met confirmed around 150 people have been arrested so far over showing support for Palestine Action, which was banned by the Government as a terrorist organisation, and for assaulting police officers.
The force added: "Officers policing the Defend Our Juries protest in Parliament Square have been subjected to an exceptional level of abuse including punches, kicks, spitting and objects being thrown, in addition to verbal abuse.
"Any assaults against officers will not be tolerated and arrests have already been made. We will identify all those responsible and prosecute to the full extent of the law."
But a DOJ spokesperson called it an "astonishing claim" from the police.
I think they were probably embarrassed and maybe making this claim about violence to try and sort of deflect and change the narrative away from what the real story is here, which is that this ban is not enforceable and there's just too many of us
They told the PA news agency: "I've been here all day and I haven't seen any violence or aggression from anyone.
"The people they're arresting, it's all people sitting here silently holding signs, a lot of elderly, many disabled people, veterans, vicars, healthcare workers in their scrubs, just like the picture of peaceful protest. I've only seen aggression and violence from the police."
"They've made so few arrests, it was going so slowly, I think they were probably embarrassed and maybe making this claim about violence to try and sort of deflect and change the narrative away from what the real story is here, which is that this ban is not enforceable and there's just too many of us," they added.
"It's just being made a mockery of, really, by the numbers of people who are defying it and will continue to defy it until the Government drop it, and now there's a new Home Secretary I think there's a real opportunity for it to be dropped and this sends a strong message on her first day since her appointment that she's inheriting this ban which is just unenforceable."
There were frantic scenes in the area throughout the afternoon, as officers drew their batons during clashes, forced their way through crowds carrying arrested protesters and had screaming arguments with demonstrators.
One protester was seen with blood streaming down his face and several others fell over in a crush at one point.
A man, who appeared to be in his 50s, unfurled an Israeli flag in front of demonstrators, sparking a scuffle as protesters tried to seize it.
Police rushed in and escorted him away as a small crowd followed, calling him a "baby killer", "paedophile" and "Zionist", and chanting "arrest him now".
Protesters also gathered for a separate demonstration in the capital organised by the Palestine Coalition - which includes the groups Palestine Solidarity Campaign and Stop the War - while rallies were held in Belfast and Edinburgh.
Palestine Action was banned as a terror organisation in July after the group claimed responsibility for an action in which two Voyager planes were damaged at RAF Brize Norton on June 20.
The Home Office is set to appeal against the High Court ruling allowing Palestine Action's co-founder, Huda Ammori, to proceed with a legal challenge against the Government over the group's ban.
Ms Ammori took legal action against the department over then-home secretary Yvette Cooper's decision to proscribe the group under anti-terror laws, which made membership of, or support for, the direct action group a criminal offence punishable by up to 14 years in prison.