General secretary Iain McNicol said membership was “now over 325,000 and rising”
“A warm welcome to the 15,500 who have joined in the last 24 hours,” he wrote on Twitter.
Newly-elected deputy leader Tom Watson said he hoped many of the more than 100,000 who had paid £3 to vote as a “registered supporter” in the campaign would upgrade to full membership.
“I hope they can come with us on the journey to the election in 2020,” he told BBC1’s Andrew Marr Show.
“Let’s get these new members involved in campaigning, helping relay our roots in communities, being involved in a digital revolution in the party that allows members to feel that they’re more included in the decisions we make.
“We want them to be part of that and I don’t think anyone is disagreeing with that,” he said - ruling out any move to convert them automatically on grounds of practicality and cost.
Mr MacNicol said yesterday that he looked forward to seeing new supporters “out on the doorsteps, alongside the members who’ve campaigned for years, or even decades”.
“I want every supporter who voted in this election to do more than click a button and part with £3.00. I want you to join our campaigns, create lively, visible Labour Parties in every community.
“We know from the election in May how vital activism is. The disappointment of defeat weighs heavy in all our hearts.”
Labour’s membership also rose significantly in 2010 when Ed Miliband became leader, but slipped back before reforms to the party’s structures such as voting rights.