"Jewdas" the group organizing the event describes itself as providing a "radical voice for the alternative diaspora" and is well-known for its outspoken views on the Jewish State.
Mr Lansman, who is Jewish, said: "It was his night off, he had nothing in his official diary, his office didn't know he was there".
'So I don't think this is as significant as it's being made out'. Last week, a rally of more than 1,500 people outside of Parliament organized by mainstream Jewish groups said that "enough is enough", and called on the leader to do more.
Mr Arkush also claimed Jewdas are "not all Jewish" and branded the group "lifelong campaigners against the Jewish community to whom they show the upmost disregard and contempt".
Charity leader Ronnie Harris told Jewish News he left the party amid the latest storm over its handling of allegations of anti-Semitism.
Charlotte Nicols, who attended the Jewdas seder alongside Corbyn, defended his presence.
However the Campaign Against Antisemitism said Corbyn's "Jewdas" Seder attendance was a "very clear two-fingered salute [British curse] at mainstream British Jewry".
The group named Jewdas have previously claimed anti-Semitism allegations against the leader as "the work of cynical manipulations by people whose express loyalty is to the Conservative Party and the right wing of the Labour Party".
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Momentum has said accusations of anti-Semitism "should not and can not be dismissed simply as right-wing smears" and that unconscious anti-Jewish bias is "more widespread in the Labour party than many of us had understood even a few months ago".
Jonathan Goldstein told BBC Radio 4's Today programme: "Look, firstly, I think it's important to say that every community is open to all strands of opinion, we have no issue with Jewdas and Jewdas having its opinions that it does, no community should have any problem with that".
Karen Pollock, the chief executive of the Holocaust Educational Trust, claimed that Corbyn's meeting was "clearly deliberate".
Corbyn had to apologise to the Jewish community in March after Jewish leaders wrote an open letter to the Labour leader accusing him of "siding with antisemites". "Please feel free to make a donation!"
"[The Seder] reminded us that the reasons we're Jewish are the same as our reasons for being revolutionaries: our commitment to making this world better than it is now".
Jewdas has dismissed anti-Semitism allegations as right-wing smears.
Criticism of the Labour leader reached a crescendo last week when 1500 demonstrators protested outside Parliament Square in central London calling for severe punishments against antisemitic members of the party.
'Jeremy was invited to a Passover Seder with Jewish members of his local community, ' said the spokesman.
Ms Formby, an ally of Mr Corbyn, also vowed to introduce "new procedures to deal with complaints and disciplinary cases".