Besigye's Forum for Democratic Change (FDC) party said he was held after checking on reports of an illegal tally centre in his home district manned by the ruling National Resistance Movement party.
"He walked in and saw them pre-ticking ballot papers. They arrested him and took him to an unknown place," said senior FDC official Shawn Mubiru.
Besigye's lawyer, Ladislaus Rwakafuzi, later said he had been "released without charge and taken home," in line with a past pattern of detention and release.
"What happened is that Besigye stormed a police facility where we operate and receive emergency calls... Besigye wanted to force his way inside the facility with media and ended up assaulting a police officer," Onyango said.
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"The officer has opened up a case of assault against Besigye and we shall open a case of criminal trespass against Besigye because he wanted to enter without permission".
The voting, for the presidency and parliament, sees incumbent head of state Yoweri Museveni facing a challenge from seven candidates.