A string of Labour moderates have lined up to warn that the party could die if it does not change.
The warnings came a day after Jeremy Corbyn's re-election as leader, as an ICM opinion poll for The Sun on Sunday put Labour on 26% - its worst standing since 2009 and 15 points adrift of the Conservatives on 41%.
Delegates at the party's annual conference in Liverpool waited to see whether any of the MPs who walked out of Corbyn's shadow cabinet in June would respond to his calls for unity by agreeing to return to the frontbench.
Corbyn has said he wants to offer an olive branch to his critics after strengthening his mandate with a 62%-38% victory over challenger Owen Smith.
But he has set his face against their key demand for MPs to elect the shadow cabinet, and the ruling National Executive Committee kicked the issue into the long grass by agreeing to consider the issue at an awayday on November 22 - long after a possible reshuffle is likely to take place.
Corbyn has been in talks with key backbench figures but has yet to announce any appointments to fill numerous holes in his frontbench team, and shadow chancellor John McDonnell said there may be no changes before MPs return to Westminster next month.
Meanwhile, Labour peer Lord Mitchell quit the party, saying it was "very difficult" for Jews who support Israel to remain members after Corbyn's re-election.
Defiant moderates attended a packed rally of the Labour First movement to hear warnings that the party's future existence is at risk as well as pleas for centrists to stay and fight rather than quit or defect in protest at Corbyn's re-election.
As numbers overfilled the venue, speakers went outside to address crowds spilling out onto the street.
Former shadow cabinet minister Vernon Coaker warned: "The political terms of trade in this country are changing. The Labour Party has to change. Our policies have to change.
"If we don't change we will die."
Hilary Benn, who was sacked as shadow foreign secretary by Corbyn, told the gathering: "Don't be disheartened because in the end the values that bind us together will win."
And Angela Eagle, who mounted a brief challenge for the leadership after the revolt within the party's ranks at Parliament, was given a standing ovation as she said she had received 47,000 pieces of abuse simply for changing her Facebook photo.
Labour MP Ruth Smeeth, who has been targeted with death threats, said: "If I'm not going anywhere, not one of you gets to go anywhere because we are staying and we are fighting."
At a fringe meeting within the conference, former shadow energy secretary Lisa Nandy warned that traditional supporters were "moving away" from the party, which risked following the old Liberals into irrelevance if it "stands still while the world around (it) changes".
And London Mayor Sadiq Khan told the Sunday Times: "We know from history - the Gang of Four in the 1980s - that when the Labour Party splits, we are out of power for a generation. Now it is far more serious than that. If the Labour Party splits, it could be the end of the Labour Party."