Uganda's opposition leader Kizza Besigye and several other politicians have been arrested shortly before they were due to attend a Kampala rally.
The BBC's Joshua Mmali says police used tear gas on the group, before taking them to a police station.
Mr Besigye had been meeting his allies in advance of a protest against corruption and economic hardships.
Last year, Mr Besigye was arrested several times during clashes between security forces and his supporters.
They were protesting over what they said was a rigged election in February 2011, as well as the rising cost of basic foodstuffs.
Our correspondent says earlier in the day, police had deployed around the homes of prominent politicians to prevent them from leaving their homes.
The police have yet to say why Mr Besigye and the others are being detained.
The opposition says President Museveni's government's "failed economic policies" have caused the cost of living in Uganda to spiral out of control.
Mr Besigye was previously President Yoweri Museveni's personal doctor, but the pair fell out.