More than 32,000 people have joined Labour since the general election, including 10,000 former Liberal Democrat supporters, the party's acting leader said today.
Harriet Harman said there had been an "unprecedented surge" in membership, including many from Liberal Democrats "dismayed" that their vote had helped put the Conservatives into power.
Votes cast in council elections since May were 45,000 for Labour, 34,000 for the Tories and 25,000 for the Liberal Democrats, she told the TUC Congress in Manchester.
Ms Harman said more than 700 women were registered to attend Labour's annual conference later this month, when she will hand over to a new leader a growing party on "sound" financial footing.
"These last few months have been an extraordinary period for British politics and for the Labour party. We may have been defeated in the election, but we are not defeated in our spirit."
Labour leadership candidate Andy Burnham said today he would like a review of the party's voting system to elect leaders and deputy leaders to give ordinary members a greater say.
"At the moment, the votes of MPs and MEPs have a much greater weighting than those of ordinary members. Meanwhile, there are many members who have more than one vote, by virtue of their union or affiliate membership," said Mr Burnham.
"I want to work towards a true 'one member, one vote' system, which strengthens our trade union link by bringing grassroots union members into party membership, and which does not say that my vote as an MP is worth 600 times the vote of an ordinary party member.
"That's how we build a true mass membership party."
Former deputy prime minister John Prescott appeared to hint today that he favours David Miliband as Labour's next leader, following speculation that he was leaning towards his younger brother Ed.
Lord Prescott is not formally endorsing any of the five candidates, because he is standing in a separate election for the post of party treasurer.
But on his blog today, he reminded readers of a post he made following last week's televised Sky News hustings, in which he said that the new leader "will need to highlight Labour's achievements alongside their new progressive policy agenda and campaign hard to promote both".
He added: "On this occasion, it seems the clearest defence of the Labour government's real achievements over 13 years came from David Miliband.
"David said: 'We have to defend it with an absolute passion because if we trash our record, no one's going to believe us in the future.'
"David has also rightly made much of the importance of greater campaigning and organisation.
"So it would be good if all the candidates during future television debates and meetings make greater prominence of our achievements."
In today's post, Lord Prescott added: "I still stand by these comments."