Police in northeastern Uganda fired tear gas on Wednesday to disperse a campaign rally held by Amama Mbabazi, the country's former prime minister, who is seeking to unseat veteran leader Yoweri Museveni in next year's elections.
Museveni, who has ruled the east African nation for nearly 30 years, is expected to face a tough challenge from Mbabazi, a former ally and influential figure in the ruling National Resistance Movement (NRM) party before he quit.
Since Monday, Mbabazi has been holding large rallies across eastern Uganda to gauge support for his candidacy and to rally supporters. The government has dismissed the gatherings as "illegal."
Josephine Mayanja-Nkangi, a spokesperson for Mbabazi, said police fired tear gas shortly after his supporters gathered at a sports ground in Soroti, a town about 300km (190 miles) from capital Kampala.
"They fired tear gas to disperse our rally and without any provocation or any cause at all," she said.
Police did not answer requests for comment.
Reports in local media said military aircraft had also hovered over the grounds where Mbabazi's supporters were assembled, shortly before the tear gas was fired.
Shaban Bantariza, the deputy government spokesperson, told Reuters that Mbabazi's rallies were illegal because the electoral commission had authorized Mbabazi to hold "consultative meetings," not "mass rallies."
"Someone is only allowed to hold a mass rally and distribute campaign materials only when they have been duly nominated as a candidate," he said.
Opposition politicians fear the authorities will rely on a range of tactics, including intimidation by security forces, to check growing support for Museveni before the presidential election, scheduled for February and March 2016.
The government has already been accused of training militias to intimidate opposition supporters, a claim the government dismissed.
Mbabazi is standing as an independent after plans to seek NRM's nomination were dropped in July amidst accusations the ruling party was frustrating his bid.