Owen Smith has labelled Jeremy Corbyn "mealy-mouthed and weak" over his response to the antisemitism crisis engulfing Labour.
Mr Smith said the fact the party had seen "a precipitous drop-off of support" from the Jewish community over the past year "should be gnawing at our insides."
The two candidates for the Labour leadership met in the final hustings of the campaign, in front of a capacity crowd at the JW3 community centre on Sunday evening.
Both candidates declined to declare themselves Zionists, but confirmed their support for Israel's right to exist.
They also pledged they would support a rule change proposed by the Jewish Labour Movement to ensure that antisemitism is specifically named as a disciplinary offence, on the same level as expressing support for another party.
The audience at times grew vocal, particularly when they felt Mr Corbyn had not properly answered a question. One questioner asked why perpetrators of antisemitism "had remained unpunished". Mr Corbyn's response, that the situation was still being investigated, were met with cries of "shame" from the crowd.
One audience member said she had read a report in a local newspaper that John McDonnell, the Shadow Chancellor, had objected to shops in his west London constituency stocking "Israeli vegetables".
Some in the audience reacted angrily when Mr Corbyn replied that, while he did not have "all the details", he thought Mr McDonnell was referring to "settlement goods".
Mr Corbyn reiterated his support for a boycott of West Bank products, while Mr Smith said he was "not in favour" of boycotts, and called any comparison of Israel to apartheid-era South Africa "unhelpful."
Marie Van der Zyl, vice-president of the Board of Deputies, asked Mr Corbyn what it would take to expel Ken Livingstone from the party. Mr Livingstone is currently suspended for claiming Hitler supported Zionism.
Mr Corbyn responded: "Ken was suspended for remarks he made, he's under investigation, due process will be followed."
Mr Smith said he suspected that Mr Livingstone "will be let back into Labour in due course. I don't think Jeremy wanted him suspended in the first place".
Karen Pollock, chief executive of the Holocaust Educational Trust, asked Mr Corbyn: "What is your defence for associating with Paul Eisen, a self-proclaimed Holocaust denier, and Jackie Walker, who accused Jews of carrying out the slave trade."
The Labour leader responded:"When it became apparent to me that Paul Eisen's views had gone in a direction I absolutely did not agree with, I declined to attend any further events" with him.
His answer was greeted with shouts of "you knew" from the audience.
About Ms Walker, a Labour activist, Mr Corbyn said: "She did make remarks which were challenged, she was suspended from party membership, she was investigated on this, representations were made which were absolutely independent of me, she was reinstated."
Mr Corbyn was seen embracing Ms Walker 10 days ago at an event he spoke at which she had helped organise.