His closest rival, Kizza Besigye, has won just 34 percent of the votes counted so far, according to Uganda's Election Commission. The full results are expected within the next few hours.
Museveni, who is into his seventies, faced seven challengers, but the former rebel fighter is expected to be re-elected in the east African country, AFP reported.
After Besigye, Museveni's next closest challenger is former prime minister Amama Mbabazi, a former ruling party stalwart whom partial results showed trailing in distant third. He has won less than two percent so far.
Besigye, who was arrested during campaigning on Monday and again on Thursday evening, was taken into custody for a third time on Friday.
Police surrounded his Forum for Democratic Change (FDC) headquarters, firing tear gas and using water cannon, before storming the building and arresting him, saying they wanted to prevent him from unilaterally proclaiming his vote score.
He was later escorted home, where he remained Saturday with security forces surrounding his house.