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.
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