Crowds gathered and long lines formed in some areas as polling station openings were delayed and voting materials were seen being delivered after the scheduled opening time on Thursday morning.
President Yoweri Museveni, 81, faces seven other candidates, including Robert Kyagulanyi, a musician-turned-politician best known as Bobi Wine, who is calling for political change.
The East African country of roughly 45 million people has 21.6 million registered voters.
Impatient crowds gathered outside polling stations expressing concerns over the delays on Thursday morning.
There have been concerns about transparency, the possibility of hereditary rule, military interference and opposition strategies to prevent vote tampering at polling stations.
Uganda's internet was shut down on Tuesday by the government communications agency, which cited misinformation, electoral fraud and incitement of violence.
The shutdown has affected the public and disrupted critical sectors such as banking.
There has been heavy security leading up to voting, including military units deployed on the streets this week.
Amnesty International said security forces were engaging in a "brutal campaign of repression", citing a November 28 opposition rally in eastern Uganda where the military blocked exits and opened fire on supporters, killing one person.
Museveni urged voters to come out in large numbers during his final rally on Tuesday.
"You go and vote, anybody who tries to interfere with your freedom will be crushed. I am telling you this. We are ready to put an end to this indiscipline," he said.
The national electoral commission chairperson, Simon Byabakama, urged tolerance among Ugandans as they votes.
Authorities also suspended the activities of several civic groups during the campaign season.
That Group, a prominent media watchdog, closed its office on Wednesday after the interior ministry alleged in a letter that the group was involved in activities "prejudicial to the security and laws of Uganda".
Veteran opposition figure Kizza Besigye, a four-time presidential candidate, remains in prison after he was charged with treason in February 2025.
