Youths burned tyres and blockaded streets in Uganda's capital Kampala on Thursday and soldiers fanned out across the city with armoured vehicles, in a show of force a day after the arrest of an opposition presidential candidate triggered deadly unrest.

Police said three people were killed and at least 34 injured on Wednesday in what they described as riots after the arrest of Bobi Wine, a pop star and lawmaker who is the leading challenger to President Yoweri Museveni in an election in January. The state-run daily New Vision said at least seven people had died.

"This is a war-like situation, so the army has to deploy," army spokesperson Brigadier Flavia Byekwaso told Reuters.

"You can see what is going on, people are being stoned, people are being killed, vehicles are being vandalised, tyres everywhere," she said. "These things are spontaneous on all streets, so police cannot handle such a situation."

Wine, whose real name is Robert Kyagulanyi, was detained on Wednesday after authorities accused him of violating anti-coronavirus measures by holding mass rallies.

Moments after Wine was detained while campaigning in eastern Uganda, spontaneous protests erupted in Kampala and several other major towns. Security personnel responded with tear gas. A journalist who spoke to Reuters on condition of anonymity said he had seen troops firing automatic rifles.

There were signs that more violence could unfold on Thursday. A witness in Kyaliwajjala and surrounding areas in Kampala's northern outskirts told Reuters he had seen youth piling tyres and wooden barricades on roads and setting them on fire.

Byekwaso, the army spokesperson, said squads of both military and police were skirmishing with protesters in different parts of Kampala. At least two witnesses in the city told Reuters some streets in the city centre were deserted.

Authorities have been rattled by Wine's surging support, especially among the youth, ahead of the January 14 presidential election. Museveni, 76, has been in power since 1986 and is seeking a sixth term.