Jewish community leaders have accused Jeremy Corbyn of siding with anti-Semites “again and again”.
The Board of Deputies of British Jews and the Jewish Leadership Council led this evening’s protest.
Protesters carrying placards reading “enough is enough” filled Parliament Square and were met by a small counter-protest by hardcore Corbynistas who claimed the issue was a “witch-hunt” against their leader and his socialist ideals.
At one stage, police had to step in as a man carrying an “End Israel apartheid” banner led to a heated row between the two groups.
Ahead of the speeches, chants of “shame on you” rang out, directed towards Corbyn supporters, while a number of scuffles and angry exchanges took place.
Jonathan Goldstein, chairman of the Jewish Leadership Council, said: “Anti-Semitism has no place whatsoever in a mainstream political party,” to which the crowd replied “Hear, hear”.
Addressing those gathered, he said: “It’s a scourge on our society, it must be rooted out.
“So we are here to say to Jeremy Corbyn: ‘Enough is enough. The time for talking is over, the time for words is over, and the time for action has begun.’”
Board chairman Jonathan Arkush told the crowd: “This is an extraordinary event. I hope never to repeat it.
“But I am moved by how many of you are here.
“We are not just disappointed about a lack of action, we are furious.”
Labour MPs John Mann, Chuka Umunna, Wes Streeting, Luciana Berger, Stella Creasy, Liz Kendall and John Woodcock were among those attending the Monday evening protest.
Communities Secretary Sajid Javid was also present, the crowd was told.
Ms Berger, Labour & Co-op MP for Liverpool Wavertree and parliamentary chair of the Jewish Labour Movement, said: “I tell you that anti-Semitism is very real and it’s alive in the Labour Party.”
“It pains me to have to stand before you and have to say that today,” she added.
She said anti-Semitism within the party had now become more “conspicuous”, “commonplace” and “corrosive”.
Protesters left at 6pm deliver an open letter to a meeting of Labour MPs and peers at which concerns about anti-Semitism are expected to be raised – although Mr Corbyn will not attend.
Ahead of the protest, Mr Corbyn wrote to both groups apologising for the “pain and hurt” caused by instances of anti-Semitism in his party.
Mr Corbyn said: “I recognise that anti-Semitism has surfaced within the Labour Party, and has too often been dismissed as simply a matter of a few bad apples.
“This has caused pain and hurt to Jewish members of our party and to the wider Jewish community in Britain. I am sincerely sorry for the pain which has been caused, and pledge to redouble my efforts to bring this anxiety to an end.
“I must make it clear that I will never be anything other than a militant opponent of anti-Semitism.”
Mr Corbyn also personally apologised for questioning the removal of a controversial mural in London.
The Labour leader said there needed to be a deeper understanding of what constitutes anti-Semitism in the Labour Party.
The latest row was triggered by a Facebook comment from 2012 when Mr Corbyn offered a show of support for the painter of a mural at the centre of an anti-Semitism row whose controversial street art was about to be painted over.
Mr Corbyn later said he sincerely regretted not looking properly at the “deeply disturbing and anti-Semitic” picture before telling artist Mear One he was in “good company” among artists who had work removed.
In an extraordinary open letter today, the two groups said there was “repeated institutional failure” to properly address Jewish concerns.
They wrote: “We conclude that he cannot seriously contemplate anti-Semitism, because he is so ideologically fixed within a far-left worldview that is instinctively hostile to mainstream Jewish communities.”