Labour and the Conservatives in the United Kingdom said the result of the Clacton by-election would not draw a line under the saga surrounding donations received by Nigel Farage.
Polls closed at 10 pm (2100 GMT) in the election which has been shunned by the other major political parties, with Farage hoping to retain his seat.
A record 34 candidates are standing. Satirical candidate Count Binface, a man wearing a metallic rubbish bin over his head, appears to be Farage's main challenger.
Labour's chair Bridget Phillipson said Farage "can't run from the truth forever."
Meanwhile, a spokesperson for the Conservative Party said the "stunt" of calling the by-election would not draw a line under it.
The Reform UK leader quit as a Member of Parliament (MP) to trigger the by-election in protest at media and parliamentary scrutiny of donations from his backers.
Farage said it was a chance to "stick two fingers up to the establishment".
Phillipson said: "Nigel Farage spent 38 days fighting a bin in his most desperate attempt yet to deflect attention from the sleaze and scandals engulfing him and his Party."
"Farage may succeed in his run-off against a bin, but he can't run from the truth forever. The list of unanswered questions about the £5 million [$6.7 million] gift he hid from the public, his support from a convicted fraudster, and the mountain of other scandals he and Reform are facing isn't going away."
"In fact, it's getting longer by the day."
"It's time to stop the excuses and desperate distractions and finally come clean with the public. If Farage has nothing to hide, he surely would have done so by now."
A Conservative Party spokesman said: "Nigel Farage called this fake by-election in a desperate attempt to draw a line under the questions surrounding his fishy £5 million donation."
"Whatever the outcome on Friday, this stunt has not done that."
The result is likely to be announced in the early hours of Friday morning. Initial returns show that turnout is down about 15% on the election in July 2024.
Farage wanted it to be a "people versus the establishment" showdown but, rather than Labour and Tory rivals, he is facing opponents including Count Binface, actor-turned-politician Laurence Fox, three from the Official Monster Raving Loony Party and an array of independents.
The contest is estimated to have cost taxpayers about £250,000.
Photographers and TV cameras covering the count have been advised by officials they will need a wide-angle lens to fit all the candidates into a single shot.
Before quitting Westminster, Farage was being investigated by the parliamentary standards commissioner Daniel Greenberg over a £5 million gift from cryptocurrency tycoon Christopher Harborne, which he received before taking his seat in the Commons.
Under Commons rules, new MPs are required to register any gifts worth more than £300 they received in the previous 12 months, except where the gift "could not be reasonably thought by others" to relate to their political activities.
That investigation was paused when Farage resigned his seat but will resume if he wins the by-election - as bookmakers have made him the firm favourite to do.
Farage could also face an investigation over support provided by George Cottrell, a long-term ally.
Cottrell reportedly recruited and paid three staff to work on Farage's social media before the general election, and has continued to allow him to use a five-storey, Georgian property he rented near Buckingham Palace.
The main Westminster parties refused to take part in this contest but could stand if the outcome of the standards investigation into Farage results in another by-election.
If he is found to have broken the rules and a suspension of more than 10 days is imposed, it could trigger a recall petition and the prospect of Farage losing his seat, forcing a second contest.
In the 2024 general election, Farage won Clacton with 46.2% of the vote and a majority of 8,405 over his Conservative rival.