The internet has been cut off in Uganda on government orders the night before the hotly contested election.
A 38-year-old singer is challenging one of Africa’s longest serving leaders in one of the world’s youngest countries.
Robert Kyagulanyi, known by his stage name Bobi Wine, says he represents the country’s younger generation, while Yoweri Museveni, 76, says he is standing for stability.
Dozens of people have been killed in the run up to the election.
Polls were due to open at 07:00 local time (04:00 GMT) but voting was delayed in several areas.
Some polling stations did not open for close to two hours and voters in the queue had grown angry and had started shouting at the polling gate officials, the BBC’s Patience Atuhaire reports from the capital, Kampala.
She said the cause for delay was not clear.
Reuters news agency reports that at the six polling stations their reporters observed delays the ballot paper had not arrived.
Polls are due to close at 16:00 local time (13:00 GMT) but will remain open until everyone in the queue at time of closing has voted, our correspondent adds.
Results are not expected before Saturday.
Planning for violence
Ugandan police promised they would deploy officers on rooftops of the capital Kampala on polling day, which they say comes after opposition activists commanded protests from rooftops in November, when more than 50 people were killed after Bobi Wine was arrested.
Earlier this week in a televised address, Mr Museveni gave a stark warning.
“If you try to disturb peace, you will have yourself to blame. The security forces, following the law, are ready to deal with any troublemaker,” he said.
Security forces cracked down on gatherings ahead of the election and dozens have been killed.
The government says the ban on gatherings was to prevent the spread of coronavirus while the opposition say it was a smokescreen for repression.
Bobi Wine and others out of the 10 opposition candidates have been arrested on several occasions.
The US cast doubt over the electoral process and withdrew its election observers after most of its accreditation requests were denied.
In response, Mr Museveni’s spokesman Don Wanyama tweeted that there were observers from the African Union and East African Community.
“I don’t remember when Uganda last sent election observers to the US,” he added.
The government has previously said the election would be free and fair.
Bobi Wine has called on voters to remain at polling stations on Thursday and use their mobile phone cameras to record the tallying process in an effort to prevent vote rigging. – BBC