Labour leadership frontrunner Jeremy Corbyn was mobbed by dozens of supporters singing the Red Flag as he arrived at the special conference being held in London to unveil the successor to Ed Miliband.
The veteran left-winger is odds-on favourite to score a shock victory over his more mainstream rivals Andy Burnham, Yvette Cooper and Liz Kendall, after entering the race in June as 200-1 outsider.
Party sources said turnout in the ballot was 76.3% of the 550,000 entitled to vote. But there was no immediate confirmation of the breakdown of voting between full party members, trade unionists and the "registered supporters" who paid £3 to secure a vote and are believed overwhelmingly to favour Mr Corbyn.
The Islington North MP, who has been on the Labour backbenches throughout his 32-year parliamentary career, entered the Queen Elizabeth II conference centre to chants of "Jez we can".
There were more low-key arrivals for shadow home secretary Ms Cooper and the most Blairite candidate Ms Kendall, who has all but accepted defeat after trailing throughout the contest.
The election on Friday of Sadiq Khan as the party's candidate for the 2016 London mayoral contest thanks in part to an influx of party activists thought to have been attracted by Mr Corbyn's policies is seen as a pointer towards the result.
Mr Khan, who comfortably saw off Baroness Tessa Jowell to secure a tilt at City Hall, was one of the MPs responsible for securing Mr Corbyn a place on the ballot paper only to ensure a wider debate, despite backing Mr Burnham as leader.
But second preferences could play a vital role in an alternative vote system which will see the lowest-ranked candidates successively eliminated and their votes redistributed until one of the contenders reaches 50%.
With few of Ms Cooper or Ms Kendall's supporters thought likely to put Mr Corbyn second, it is thought that he needs a score of 43% or more in the first round to be confident of eventual victory.
Mr Corbyn's rivals hope figures suggesting far fewer union members have taken part than helped sweep Mr Miliband to a surprise victory in 2010 could yet see the opinion polls confounded.