Kizza Besigye was arrested on Monday in Kampala, where he is holding his last rallies ahead of elections on Thursday.
Associated Press journalists saw police whisk Besigye away on a truck as he tried to address a crowd. Police also fired tear gas.
Ugandan President Yoweri Museveni, who has been in power since 1986, is running for another term. Besigye describes Museveni as a dictator and is promising to run a more efficient government.