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LONDON — Britain's opposition leader has issued a series of apologies amid claims that anti-Semitism is rife within his party.

Labour leader Jeremy Corbyn, a socialist who shocked critics including many of his own lawmakers by engineering a better-than-expected performance in last summer's U.K. election, has admitted a mural he defended in 2012 is anti-Semitic.

Labour Party leader Jeremy Corbyn Andy Rain / EPA file

Members of Parliament from across the political spectrum were among more than 1,000 people who attended a protest arranged by Jewish groups in London on Monday afternoon.

"What on earth is going wrong with our party when this kind of event even has to be considered?" Labour lawmaker John Mann said at the event.

Left-leaning Labour has faced persistent criticism in recent years following anti-Semitic comments made by party members and even lawmakers. Party investigations into many of the accusations have dragged on, while in other cases Jewish groups have expressed disappointment when members were suspended rather than expelled.

But Corbyn's leadership has left Labour neck and neck in the polls with the ruling Conservatives.

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After Monday’s rally, Corbyn released his strongest condemnation of anti-Semitism within Labour to date — the fourth statement on the topic since Friday.

“I am sincerely sorry for the pain which has been caused, and pledge to redouble my efforts to bring this anxiety to an end,” he wrote in a letter to the two main Jewish representative bodies, the Board of Deputies and the Jewish Leadership Council.

The issue flared up last week after a Labour lawmaker asked for clarification on a 2012 Facebook comment written by Corbyn questioning a decision by local officials to remove a London street mural. It depicted men in suits with big noses playing Monopoly on the backs of naked people.

Corbyn responded on Friday by saying he wished he had paid more attention to the contents of the mural, and condemning it. But that apology fell short, with one lawmaker in his party describing it as "wholly inadequate."

On Monday, Corbyn addressed the "evil" of anti-Semitism within Labour.

He added: "The idea of Jewish bankers and capitalists exploiting the workers of the world is an old anti-Semitic conspiracy theory … I am sorry for not having studied the content of the mural more closely before wrongly questioning its removal in 2012."

The mural controversy came not long after it was revealed that Corbyn had been a member of a pro-Palestinian Facebook group where anti-Semitic imagery was shared.

Members of the Jewish community protested against anti-Semitism outside the U.K. Parliament on Monday. Tolga Akmen / AFP - Getty Images

In a strikingly critical open letter to the party, two of Britain’s main Jewish representative groups said Corbyn “did not invent this form of politics, but he has had a lifetime within it, and now personifies its problems and dangers. He issues empty statements about opposing anti-Semitism, but does nothing to understand or address it.”

“He is repeatedly found alongside people with blatantly anti-Semitic views, but claims never to hear or read them,” it went on to say.

Corbyn has long been a champion of Palestinian rights, which has complicated his relationship with Britain’s Jewish community.

In 2009, Corbyn invited representatives of Hamas and Hezbollah to an event in Parliament. At the time, he called them “friends,” which he later said he regretted. Both groups are classified by the U.K. and U.S. as terrorist groups, and have repeatedly called for the destruction of Israel.

Corbyn's own political colleagues — along with Jewish community leaders — are now demanding more than words from the Labour leader.

Mann warned "the very existence of my party is at stake" if Corbyn doesn't expel anti-Semites from Labour.

Simon Round, a spokesman for the Board of Deputies of British Jews, said that Corbyn "has made promises before but we have yet to see him follow through."

Round added that he hopes the result will be different this time.