Leading opposition politician Kizza Besigye was arrested for planning anti-government riots in the Ugandan capital Kampala, police said on Saturday.
Besigye has led protests for years aimed at toppling President Yoweri Museveni, to whom he lost a presidential election in February 2011. He denounced that poll as fraudulent.
His supporters were at the forefront of widespread anti-government protests against the high cost of living in 2011 and, after a lull last year, they led demonstrations again in October and Besigye was arrested.
Besigye has lost three elections against Museveni, who has been in power for more than 26 years, and stepped down as leader of Uganda's biggest opposition party, Forum for Democratic Change (FDC) last year, but is still a popular politician.
Earlier this week police banned opposition leaders from clearing up rubbish in Kampala's suburbs, saying it was a cover for staging public protests.
"Following attempts to unlawfully assemble and carry out riotous demonstrations ... Dr. Kizza Besigye has been arrested this morning," police said in a statement.
The opposition holds a range of grievances against Museveni, accusing him of seeking to be president for life as well as failing to tackle corruption and rights abuses.
Major western donors withheld aid to the east African country of 33 million people last month citing graft claims in the prime minister's office.
FDC secretary general, Alice Alaso, told Reuters Besigye had been arrested outside his home as he drove out.
Alaso said she visited Besigye at Kira police station, about 13 km (8 miles) from the city centre, where he was being held.
Police stopped her giving him a bottle of water, she said.
"Their aim is to starve him ... how can you arrest someone when he has committed no crime?" she said.
Police did not say what charges they would bring against Besigye, who went for medical treatment in Kenya in 2011 after he was nearly blinded with chemical spray by police breaking up demonstrations.
(Reporting by Elias Biryabarema; Editing by Louise Ireland and Duncan Miriri)