Sadiq Khan has launched a scathing attack on Jeremy Corbyn as he described the election result as "catastrophe for Labour, London and the UK".
The Mayor of London called on Mr Corbyn to stand down quickly so the party could begin to "change fundamentally".
He wrote a lengthy statement on Facebook saying the party's strong performance in the capital was "no consolation" for its failure nationwide.
Mr Khan also promised to "stand up for London against Brexit over the months ahead" as well as EU citizens living in the city.
It comes as Mr Corbyn refused to take personal responsibility for Labour's loss of 59 seats as the Conservatives won a landslide majority.
The Islington North MP told journalists he had "done everything he could" to lead the party to a win, adding that Labour membership had more than doubled since he ascended to the top position.
But Mr Khan slammed Mr Corbyn as well as the party's "shocking" failure to tackle anti-Semitism and put forward a credible "set of priorities for governing".
He wrote: "If we are truly honest with ourselves, we knew in our hearts that Jeremy Corbyn’s leadership was deeply unpopular with the British people and that we were extremely unlikely to form a Labour government last night.
"Labour’s shocking and repeated failure to tackle anti-Semitism, and our inability to put forward a credible and believable set of priorities for governing have made a major contribution to the scale of this defeat.
"Brexit has accelerated a fundamental shift in British politics and rewritten our political map - the full extent of which is not yet fully understood."
He went on to say that the party now stands "more politically and culturally removed than ever before" from the people it was formed to represent.
Mr Khan said that Labour will have to "change fundamentally" to challenge Boris Johnson's Brexit, protect the union with Scotland, and deal with impending right-wing changes to the economy and public services.
"Jeremy Corbyn has said he will stand down, and this simply must now happen quickly. But the changes we have to make will not end with his leadership," Mr Khan said.
"This will not be a quick or easy job, and anyone who says they know all the answers today is not credible - whether they are blaming Brexit alone or anything else," he added.
"We must now take time to absorb and analyse the results of this election before jumping to any permanent decisions about our future."
