Jeremy Corbyn has written to Jewish leaders in the UK insisting he is a "militant opponent" of anti-Semitism.
The British opposition leader apologised for "hurt and pain" caused by instances of anti-Semitism in Labour as he faced a wave of criticism from within the Jewish community there.
The letter to the UK's Board of Deputies and Jewish Leadership Council came after the organisations launched a stinging attack on Mr Corbyn, accusing him of siding with anti-Semites "again and again".
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 British Labour Party.
He said: "Sometimes this evil takes familiar forms - the east London mural which has caused such understandable controversy is an example. The idea of Jewish bankers and capitalists exploiting the workers of the world is an old anti-Semitic conspiracy theory.
"This was long ago, and rightly, described as 'the socialism of fools'. I am sorry for not having studied the content of the mural more closely before wrongly questioning its removal in 2012."
The Labour leader said the party had been to slow to act in the past.
"I acknowledge that anti-Semitic attitudes have surfaced more often in our ranks in recent years, and that the party has been too slow in processing some of the cases that have emerged. Early action has nevertheless been taken, and we will work to speed up procedures, to deal with cases of anti-Semitic abuse or attitudes."
Mr Corbyn said criticism of Israel in relation to the "continuing dispossession of the Palestinian people" cannot be avoided.
Adding: "Nevertheless, comparing Israel or the actions of Israeli governments to the Nazis, attributing criticisms of Israel to Jewish characteristics or to Jewish people in general and using abusive phraseology about supporters of Israel such as "Zio" all constitute aspects of contemporary anti-Semitism."
In their original letter, Jewish leaders took aim at Mr Corbyn personally, saying he was "repeatedly found alongside people with blatantly anti-Semitic views" but "claims never to hear or read them".
The organisations were protesting outside the British Parliament today before delivering 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 is not expected to attend.
In their letter they said: "Today, leaders of British Jewry tell Jeremy Corbyn that enough is enough."
They said there was a "repeated institutional failure" to properly address Jewish concerns and to tackle anti-Semitism.
"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."