The arrests as of 9 pm local time (2000GMT) are in relation to the protest organized by Defend Our Juries in support of the "proscribed terrorist organisation Palestine Action," Metropolitan Police said in a statement.
Officers "were punched, kicked, spat on and had objects thrown at them" during the protest, according to the statement that said more than 25 individuals have been arrested for assaults on police and other public order offences.
Police added that more than 2,500 officers were deployed across London to manage other protests as well as the Palestine Action solidarity protest.
"The tactics deployed by supporters of Palestine Action in their attempt to overwhelm the justice system, as well as the level of violence seen in the crowd, required significant resource which took officers out of neighbourhoods to the detriment of the Londoners who rely on them," Deputy Assistant Commissioner Claire Smart said in the statement.
Hundreds gathered in Parliament Square, where demonstrators carried Palestinian flags and signs while chanting: "Over 100,000 dead and you are arresting us instead" and "Resisting genocide is not a crime."
Banners read: "I oppose genocide, I support Palestine Action" and "It’s not a crime to act against genocide."
Around 1,500 demonstrators are estimated to take part in the rally.
The Defend Our Juries group wrote on the US social media platform X that one of those arrested is Mike Higgins, a blind activist in a wheelchair.
In a similar protest last month, police detained more than 500 people.
Palestine Action was banned in July under the UK’s Terrorism Act, a decision that has led to the arrests of hundreds of pro-Palestinian protesters for holding signs or showing support for the group.
