Erias Lukwago said a tear-gas grenade was thrown into his vehicle, causing him to collapse, and police then picked him up and put him in a van where officers hit him, tore his clothes and accused him of inciting violence, the BBC reported Thursday.
Police denied mistreating Lukwago, who is a critic of President Yoweri Museveni.
In another incident, opposition leader Kizza Besigye was arrested for conducting a rally in support of Lukwago without a permit, police said.
An aide to Besigye said the opposition leader was in Kampala on business when a number of people gathered around his car, then became angry when police stepped in.
Sign language interpreter at Mandela service called out as fake on Twitter
Exploding whale video goes viral on Internet