Jeremy Corbyn has said Labour must "do better" as a row continues over how the party deals with hostility to Jews.
In a Passover message, he said it was easy to denounce anti-Semitism abroad but sometimes harder to see it closer to home.
It came as Jewish Labour peer Lord Winston said Mr Corbyn had "encouraged and endorsed" anti-Semites.
Dozens of Labour politicians are urging him to suspend a senior Momentum figure amid further anti-Semitism claims.
In an open letter, the 39 MPs and peers call for Mr Corbyn to suspend Christine Shawcroft from the party's governing body after it emerged she had sent an email showing support for a council candidate accused of Holocaust denial.
After the letter was first published, four more MPs and Lord Mendelsohn added their signatures.
In the letter, they say it is "utterly wrong" and "highly offensive to the Jewish community" that she remained a member of the National Executive Committee.
Posting on Facebook, Ms Shawcroft said that she would not be seeking re-election to the NEC and that her term would end this summer.
What is Passover?
Jeremy Corbyn's message on anti-Semitism came as Jews prepared for Passover, a major festival celebrating their freedom from slavery in Egypt in about 1300 BCE.
The term derives from Moses's instruction to the Jews that they should mark their doorposts so that the last of God's plagues against the Egyptians, the death of the firstborn, would pass over them harmlessly.
On the first night of Passover, Jewish families traditionally gather for a dinner called a "seder", where the story of the exodus from Egypt is retold.
Ms Shawcroft, who is a director of the pro-Corbyn Momentum group, stepped down as head of Labour's disputes panel on Wednesday.
Speaking to the BBC, a Labour Party spokeswoman said the letter was not being ignored but that suspending Christine Shawcroft would not be a personal decision for Jeremy Corbyn as there were formal processes for such matters.
The council candidate, Alan Bull, is accused of sharing an article on Facebook which claimed the Holocaust was a "hoax". He said he had reposted the article for the purpose of debate and did not agree with it.
Ms Shawcroft said she had not been aware of the "abhorrent" online post and said she was "deeply sorry".
In a Facebook post, which was later deleted, she said the row was "being stirred up to attack Jeremy".
In a second post, she said anti-Semitism was evil and had no place in the party.
The lead signatory of the letter, the MP Siobhain McDonagh, told the BBC the group felt "very strongly" about the issue and that she and her colleagues had been "shocked to the core".
In the letter, seen by the BBC on Thursday night, the Labour figures urged Mr Corbyn to take an "initial step" in honouring his commitment to tackle anti-Semitism.
Meanwhile Lord Winston has told the BBC he is "deeply ashamed" of his own party.
He said anti-Semitism had become endemic in the party adding, "our leader who I've no doubt at all is a good person, stands by whilst Labour descends a disastrous path."
Mr Corbyn said Labour would "never be complacent" about anti-semitism.
"We remember all our Jewish brothers and sisters, who have battled against discrimination and faced the most horrific acts of violence and mass murder," he said.
He highlighted rising levels of anti-Semitism elsewhere; in Poland, where laws prohibit acknowledgment of Polish complicity in the Holocaust, and in France, where a 85-year-old Holocaust survivor was murdered.
"We all need to do better. I am committed to ensuring the Labour Party is a welcoming and secure place for Jewish people. In the fight against anti-Semitism, I am your ally and I always will be."
The BBC understands Mr Corbyn intends to announce a speeding-up of the party's disciplinary procedures.
He has repeatedly condemned anti-Semitism, particularly in the past week following fresh accusations of anti-Jewish prejudice after it emerged he sent an apparently supportive message to the creator of an allegedly anti-Semitic mural in 2012.
The remarks led to a protest outside Parliament, organised by the Board of Deputies of British Jews and Jewish Leadership Council and attended by several Labour MPs, calling on Mr Corbyn to "root out" anti-Semitism in the party.