Uganda's main opposition leader has spoken out against more than a month of house arrest since elections he claims were rigged.

Kizza Besigye, who rejected the results of last month's election won by veteran President Yoweri Museveni, said his detention had blocked him from gathering evidence on the "scandalous announcement by the Electoral Commission of somebody who had not won an election."

Observers said the cards were heavily stacked against Museveni's opponents, as the 71-year-old's grip on his party and country — and his access to state resources — meant the result was never in any doubt.

Museveni was declared winner with 61 per cent of the vote and has rejected claims of foul play.

"It has now been a month since I was illegally detained by the Ugandan police," Besigye said in a video message posted Wednesday night, titled "32 days: My life in illegal detention".

A long-standing opponent of Museveni, Besigye has been frequently jailed, accused of both treason and rape, teargassed, beaten and hospitalised over the years, but this is the longest period he has ever been under house arrest.

"It is of course extremely stressful in detention that is undefined, that one does not know when it will end, that has no reason, that is a complete blatant violation of one's rights," he added, wearing a t-shirt with the slogan "I can't breathe".

Besigye first tweeted a picture of himself wearing the t-shirt earlier this month, to which government spokesman Ofwono Opondo replied, "So, you can't breathe, how are you alive then?"

In his video message Besigye said, "It has been very frustrating that we have been sitting here and seeing the injustice being perpetuated."

Judges have begun hearing a legal challenge to the results, launched by third-placed presidential loser, ex-prime minister Amama Mbabazi.

READ: Mbabazi lawyers poke holes in Uganda results tallying system

"I try to use my time as best as I can," Besigye said, saying he was doing "a lot of reading" and that visitors were now allowed into his house.

"We have demanded for an audit for the election, which we believe will confirm our victory, which if they do not cooperate we will go ahead and seek to enforce using and relying on our supporters."

He added: "The struggle will definitely continue until the people of Uganda get what they want, are led the way they decide and by a person they decide, we are not turning back."