Uganda's main opposition leader, who was arrested in the run-up to the presidential elections last week, was re-arrested on Monday as he was about to lead a procession through the capital, Kampala.
Kizza Besigye was picked up and he left his home in the north-eastern suburb of Kasangati and taken to Naggalama police station, about 20 kilometres east of Kampala.
The station is heavily guarded by anti-riot police and reporters have been refused entry to the facility.
Museveni, 71, received 60 per cent of the 9.7 million votes cast in Thursday's election, which international observers said was riddled with irregularities.
Besigye, who won 35 per cent of the poll and who has been under house arrest since Saturday, announced that he would lead a procession through Kampala on Monday to fetch the election results from the Electoral Commission.
Police had relaxed restrictions on his movements and he was allowed to hold press briefings at his home.
However, the police warned on Sunday evening that "necessary means" would be used if Besigye went ahead with leading the procession.
"We therefore urge Dr Kizza Besigye to respect the law. Failure to comply will leave the police no option but to use necessary means to maintain law and order in accordance with the constitution," police spokesman Fred Enanga said in a statement.
Violence erupted in the capital Kampala on Friday, a day after the presidential and parliamentary elections, when police stormed Besigye's party offices, arrested him with several others and charged him with illegally running a parallel vote-counting centre.
The arrest sparked widespread rioting in the capital. Gunfire was heard, tear gas was used and 18 people were reported to have been injured, 10 of them seriously.
Museveni came to power in 1986 following a guerrilla campaign. He has won four elections since 1996.