Uganda's opposition leader Bobi Wine said the country's election faced "widespread fraud and violence", as votes trickled in on Friday under an internet blackout.
The 38-year-old former pop star-turned-MP did not give details about his accusations, which contradicted the government's account that Thursday's vote had been largely peaceful.
The internet was down for a third day as vote counting continued, with provisional results from 24 per cent of polling stations giving President Yoweri Museveni an early lead of 65 per cent while Wine trailed with 27 per cent.
Wine, who galvanised young Ugandans with his call for change, said in a tweet early on Friday that he was confident of victory despite "widespread fraud and violence" during the poll. He gave no further details.
The capital Kampala was quiet and some businesses were closed, while soldiers and police patrolled on foot the day after the election.
Full results are expected by Saturday afternoon.
The election commission chief Simon Byabakama assured the nation on TV after polls closed on Thursday that results were arriving at the national tally centre despite the nationwide internet blackout.
"We are not using local internet to transmit our results, we are using our own system," he said, without giving details of that system. "Don't worry, results will come."
The government ordered an internet blackout until further notice on Wednesday, a day after banning all social media and messaging apps.
Mr Museveni is seeking a sixth term in office, having ruled for almost four decades, and his main rival among 10 opposition candidates is Wine, whose popularity among a youthful population has rattled the former rebel leader.
Voting in Kampala took place under a heavy security presence of soldiers and riot police in the streets and at polling stations.
The election followed one of the most violent campaigns in years, with harassment and arrests of the opposition, attacks on the media and scores of deaths.
Mr Byabakama said the vote had gone off in a "peaceful and tranquil manner", and police spokesman Fred Enanga said there had been "no major cases of violence reported".
But a senior foreign diplomat told AFP there had been sporadic incidents of violence and many irregularities but no sign of mass manipulation.
The United States, European Union, United Nations and global rights and democracy groups raised concerns about the integrity and transparency of the election.
Only one foreign organisation, the African Union (AU), sent monitors, along with an AU women's group.
On Wednesday, the US, a major aid donor to Uganda, said it was cancelling a diplomatic observer mission after too many of its staff were denied permission to monitor the election.
Wine promised non-violent street protests should Ugandans feel the election was stolen.
Mr Museveni said that using violence to protest against the result would amount to treason.
He has ruled Uganda since seizing control in 1986, when he helped to end years of tyranny under Idi Amin and Milton Obote.
Once hailed for his commitment to good governance, the former rebel leader has crushed any opposition and tweaked the constitution to allow himself to run again and again.
And for many in the country, where the median age is 16, Mr Museveni's glory days are no longer relevant or sufficient.
But the 76-year-old, one of Africa's longest serving leaders, has never lost an election and observers expect this time around will be no different.
His opponents – most visibly Wine, who spent much of the campaign in a bulletproof vest and combat helmet – were arrested, blocked from rallying and dispersed with tear gas throughout the campaign.
Two days of protests in November led to the deaths of 54 people.