LONDON — Thousands of people including former British Prime Minister Boris Johnson gathered in London on Sunday for a march against antisemitism, a day after large crowds turned out for a pro-Palestinian rally.

Johnson was joined by the U.K.’s Chief Rabbi Ephraim Mirvis and other senior government officials at the march to express solidarity with the Jewish community. Organizers billed it as the largest gathering against antisemitism in London for decades.

Marchers waved Israeli and the U.K.’s Union flags and held placards reading “Never Again Is Now” and “Zero Tolerance for Antisemites.”

Stephen Yaxley-Lennon, the former leader of the far-right English Defence League, was detained by police at the march. Yaxley-Lennon, more widely known by his alias Tommy Robinson, was among crowds of counter-protesters who clashed with police during an Armistice Day march in London earlier this month.

Police said he refused to leave after he was warned about concerns that his presence would cause “harassment, alarm, and distress to others.”

Sunday’s march was organized amid concerns about rising tensions sparked by the Israel-Hamas war in Gaza.

Gideon Falter, the chief executive of Campaign Against Antisemitism, said that the rally came after weeks of pro-Palestinian protests that had made the capital a “no-go zone for Jews.”

He said antisemitic incidents in the U.K. have surged since the war began, and condemned what he called “appalling” placards seen at the protests – including ones “showing a Star of David thrown in the bin with a caption that says ‘please keep the world clean.’”

On Saturday, tens of thousands of pro-Palestinian protesters marched to demand a permanent cease-fire in the war.

Police said the majority protested peacefully, but 18 people were arrested for offenses including suspicion of inciting racial hatred.

The Stop the War coalition, which organized Saturday’s rally, stressed that those taking part oppose racism, antisemitism, and Islamophobia.

During the Nov. 11 Armistice Day demonstration in London, pro-Palestinian protesters marched peacefully but far-right counter-protesters whom police described as soccer “hooligans” clashed with officers trying to prevent them from attacking the march.

Organizers of Sunday’s march said it was the largest gathering of its kind since 1936 when hundreds of thousands of people blocked a planned march by the British Union of Fascists through a Jewish neighborhood.