Ugandan police on Thursday arrested two top opposition leaders and presidential candidates hoping to challenge veteran President Yoweri Museveni in elections next year, police said.

Ex-prime minister Amama Mbabazi was arrested in central Uganda, while Kizza Besigye, a leader of the Forum for Democratic Change (FDC) party, was arrested at his home just outside the capital Kampala.

Both were understood to have been arrested ahead of campaign rallies they had planned.

Museveni, who has led the east African nation since 1986, has been endorsed by the ruling National Resistance Movement (NRM) to represent it in next year's election.

But Mbabazi, a former close ally of the president, announced last month he would challenge the 70-year-old incumbent for the nomination.

The move sparked criticism from the government, which pointed out that many of the failings of which Mbabazi accuses Museveni happened when he was the party's secretary general.

Police spokesman Fred Enanga said Mbabazi had been stopped from holding an "unlawful" meeting, as it had not been cleared by the electoral commission, and therefore officers therefore made "preventative arrangements" to block the rally.

"He defied our directive and he has been arrested," Enanga said, adding that his daughter was also being held. "If there is evidence, we shall charge him."

Mbabazi's wife Jacqueline told AFP her husband had expected the arrest.

"It was expected of this government... this action goes to prove the dictatorship we have in our country, where people are not free to exercise their rights," she said.

Police have made several warnings against holding public rallies without permission, with organisers required to submit meeting details to the police at least three days before.

Police also confirmed Besigye -- a three-time presidential election challenger and a former personal doctor to Museveni -- was being held in Kampala.

After he lost elections in 2011, Besigye led repeated anti-government protests, at which he was regularly arrested and held for a few hours before being released.

Besigye's colleague, FDC president Mugisha Muntu, a retired army general, has also said he will run.

Last month police also briefly arrested, and then released with caution, outspoken general David Sejusa, who is also expected to run in presidential elections.

Sejusa, a former intelligence chief, was arrested for unlawful assembly after motorbike taxi drivers spotted the general in his car and followed him, as though in a rally.

Museveni on Thursday left for neighbouring South Sudan to celebrate that country's fourth independence day.

The celebrations are taking place under the shadow of an 18-month civil war between government forces backed by thousands of Ugandan troops and rebels led by a former vice president.

Museveni was this week also appointed as the regional mediator to try to broker an end to months of violence in Burundi, sparked by President Pierre Nkurunziza's controversial bid for a third term in power.