Anti-riot police clashed with members of Forum for Democratic Change party offices in Najjanankumbi in Kampala, after final presidential election results were announced.
FDC officials at the headquarters urged supporters not to be confrontational and that the party had no plans for violence.
Police authorities speaking at Nambole stadium said forces deployed around the city were a precaution against violence.
However, the deployment of the forces at the FDC headquarters was unnecessary, said FDC spokesman Major Rubaramira Ruanga.
"Why they're doing it, we really don't know," he said.
"We're not intersted in a war."
Later, FDC candidate Kiiza Besigye said he rejected the election result announced by the electoral commission that gave incumbent President Yoweri Museveni victory.
The commission announced that Museveni had received 59 percent of the vote compard to Besigye's 37 percent.