Mr Hughes, MP for Southwark North and Bermondsey, emerged during the eight-week contest as the nearest rival to Mr Kennedy, but conceded that he was unlikely to win.
One unofficial estimate yesterday put the turnout at about 60 per cent among the party's 90,000 members, with no evidence of a last- minute surge in votes.
Party insiders blame the turnout on the fact that many members are on holiday.
All five candidates - Mr Hughes, Mr Kennedy, Jackie Ballard, David Rendel and Malcolm Bruce - now have to endure an agonising weekend until the result is declared on Monday.
Mr Kennedy, MP for Ross, Skye and Inverness West, and the strong favourite, is thought to have polled most votes . But the party's agriculture spokesman could be overtaken by Mr Hughes under the proportional representation system used in the contest.
If none of the candidates gains more than 50 per cent of votes on the first round, the second preferences of voters backing the least popular candidate will be redistributed among the remaining field.
The Hughes camp believe that he will garner most second preferences and could go on to beat Mr Kennedy, but they are not making any predictions of the outcome.
As the only woman in the race, supporters of Ms Ballard, the MP for Taunton, hope she will benefit from second preferences.
But the other contenders, Mr Bruce and Mr Rendel, are expected to poll poorly.
Mr Kennedy entered the campaign as the clear front-runner to replace the party leader Paddy Ashdown but has had to distance himself from extending links with Labour in response to complaints from party activists.
Mr Hughes is clearly the activists' favourite, but the race will be decided by the "armchair members", many of whom are assumed to favour the telegenic Mr Kennedy.
Mr Hughes said yesterday that he wasn't likely to win, but made clear that he expected a senior post in the party if he polled well.
"I do not expect to win. Simon Hughes is still not the bookies' or the journalists' favourite, but the great joy is that it is for our members to decide," he said. "Whoever wins, all the other candidates will, I am sure, serve happily and loyally under the new leadership."
The postal ballot of the party membership will be counted by the Electoral Reform Society on Monday morning and the result will be declared a few hours later at the Royal Commonwealth Society in London at 3pm.
Bookmakers William Hill are no longer taking bets on the contest. The latest odds were Charles Kennedy 2/9; Simon Hughes 3/1; Malcolm Bruce 12/1, David Rendel 25/1 and Jackie Ballard 50/1.
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