Labour is "united" in opposing anti-Semitism, Jeremy Corbyn insisted as the row over the party's handling of controversial comments by prominent figures continued to cause serious unrest.
The Opposition leader said he and the party "stand absolutely against racism in any form" as senior allies accused internal critics of whipping up a false "crisis" to undermine his position.
"We stand united as a Labour movement recognising our faith diversity, our ethnic diversity, and from that diversity comes up strength," he declared as he addressed a May Day rally in London.
Mr Corbyn announced an independent review and pledged to tighten party codes of conduct in a bid to put a lid on the furore - which has seen MP Naz Shah and close ally Ken Livingstone suspended.
But he faced calls from Israeli politicians and diplomats to give a more "unequivocal" condemnation and warnings - including from the party's London mayoral candidate Sadiq Khan - that the party would be punished in the May 5 elections.
Sunday newspapers were full of speculation that MPs were coming closer to launching a challenge to his leadership - with a poor showing at the ballot box or a vote in favour of Brexit potentially sparking an internal coup.
Opponents have accused him of acting too slowly to deal with incidents - most notably Mr Livingstone's incendiary assertion, while defending Ms Shah, that Hitler was a Zionist before he "went mad and ended up killing six million Jews".
But allies launched a ferocious fightback and warned critics they had no chance of ousting the leader.
Shadow cabinet minister Diane Abbott said it was "a smear to say that the Labour Party has a problem with anti-Semitism".
Mr Livingstone's comments linking Hitler with Zionism - for which he has declined to apologise in a string of media interviews - were "extremely offensive", she told BBC1's Andrew Marr Show, but not part of any wider pattern.
"Two hundred thousand people have joined the Labour Party. Are you saying that because there have been 12 reported incidents of hate speech online, that the Labour Party is somehow intrinsically anti-Semitic?"
She said she would be "dismayed if some people were hurling around accusations of anti-Semitism as part of some intra-Labour Party dispute".
Unite union leader Len McCluskey said Mr Corbyn was the victim of "a cynical attempt to manipulate anti-Semitism for political aims" that was "got up by the right-wing press aided and abetted by Labour MPs".
"Once the mood music of anti-Semitism dies down, then next week and the week after there will be another subject," he told BBC Radio 5 Live. Pienaar's Politics.
The row had been ignited by party grandees who "get out of their wheelchair and toss a few hand grenades in", he said.
A poll carried out as the controversy unfolded gave the Conservatives an eight-point lead, and experts tip Labour to lose up to 150 council seats in England and face a hard night in elections to the Scottish and Welsh governments.
Israel's new ambassador to Britain said parts of the left were "in denial" about anti-Semitism and criticised Mr Corbyn's links to groups such as Hezbollah and Hamas.
"There has to be an unequivocal message from leadership saying that there is no solidarity with anti-Semites," Mark Regev said.
The leader of the Labour opposition party in Tel Aviv said recent events needed to act as a "red alert" that urgent action was required.
In an open letter to his UK counterpart, Israel's Labour leader Isaac Herzog invited Mr Corbyn to lead a delegation to the Yad Vashem Holocaust memorial in Israel to help "engage and better understand the scourge of anti-Semitism".
The Board of Deputies of British Jews said: "We hope that the new inquiry will mark the beginning of a more certain and clear Labour attitude against anti-Semitism, from the leader to the grassroots, including clarity about people and groups with whom it is not appropriate to share panels."