A prominent opposition figure in Uganda has been detained following violent clashes Monday night that allegedly started when the president's motorcade was pelted with stones, a military official said Tuesday.

Lawmaker Kyagulanyi Ssentamu was held overnight in a police cell in the northwestern town of Arua, where he and other politicians, including President Yoweri Museveni, had been campaigning in an election for a lawmaker, said Capt. Jimmy Omara, a spokesman for the Special Forces Command.

Ssentamu, a popular pop singer and a self-described ghetto child who was elected to the National Assembly last year, has emerged as a powerful voice in local politics with his calls for the young people to "stand up" and take over this East African country from what he says is the failed leadership of the current government. Still in his 30s, many of Ssentamu's followers see him as a future leader and are urging him to run in the next presidential election in 2021.

Lawmaker Allan Ssewanyana, a close ally of Ssentamu's, said he was concerned for his colleague after being unable to reach him by phone.

Ssentamu said on Twitter Monday night that his driver was shot dead by the police "thinking they've shot at me." He posted a photo of a bloodied man slumped in his car seat.

Maria Burnett of Human Rights Watch urged authorities to investigate and "arrest those responsible - no matter who they are."

Police spokesman Emilian Kayima said an unidentified man had been killed as the security forces tried to "calm down the situation" after Museveni's convoy came under attack from opposition supporters throwing stones.

The election in Arua is being held because the area's member of parliament was shot dead near the capital, Kampala, earlier this year.

That killing, and many others of prominent people in recent times, remains unsolved. Uganda is experiencing a spike in gun attacks often blamed on unidentified assailants.

Museveni, a key U.S. security ally, took power by force in 1986 and has since won election four times. The last vote in 2016 was marred by allegations of fraud.

Although Museveni has campaigned on his record of establishing peace and stability in Uganda, some worry that those gains are being eroded as he stays longer in power. Museveni, who is 73, is now able to seek re-election in 2021 because parliament passed legislation last year that removed remove a clause in the constitution that had prevented anyone over 75 from holding the presidency.

Uganda has not witnessed a peaceful transfer of power since independence from Britain in 1962.