A huge crowd of over 300,000 people showed their solidarity with the Palestinians in a march through central London on Saturday, while police detained more than 120 people who tried to disrupt the rally with far-right counter-protests.

The police clashed with the far-right groups who opposed the march, which coincided with Armistice Day, the day when Britain honours its fallen soldiers from World War One and other conflicts.

The prime minister, Rishi Sunak, denounced the violence that occurred at the Cenotaph war memorial and also criticised the “Hamas supporters” who participated in the larger rally, “chanting antisemitic slogans and waving pro-Hamas banners and clothes on the protest today”.

The atmosphere was already tense before Saturday’s rally – the largest in a series of demonstrations to express solidarity with the Palestinians and demand a ceasefire in Gaza – after the home secretary, Suella Braverman, labelled them as “hate rallies” led by “thugs”.

The Metropolitan Police of London ignored the government’s requests to stop the event, saying they did not expect serious violence, which caused tension with the authorities.

The police announced in a statement yesterday evening that they had detained 126 people so far, most of whom were right wing protesters who were part of a large group that the police said had football hooligans among them.

“The level of violence from the right wing protesters against the police today was exceptional and very worrying,” said Assistant Commissioner Matt Twist, and he also said that they found a knife and a knuckleduster during the searches.

He said the march had been preceded by a heated debate over policing and protest, which had heightened community tensions.

The main pro-Palestinian rally, which was much bigger, was peaceful, but the senior officer said some splinter groups had left it and attacked officers with fireworks, hiding their faces with masks. They were arrested.

He also said they were looking into a few cases of hate crime and supporting banned groups.

Sunak urged the police to be strict.

He said in a statement on Saturday night: “The law must deal with all crimes quickly and firmly. That is what I told the Met Police Commissioner on Wednesday, and that is their responsibility and my expectation.”

Police clashed with right wing protesters all day, using batons to control them as they threw bottles.