Renegade Ugandan army General David Sejusa has accused President Yoweri Museveni of creating a "political monarchy" to hang on to power.
In his first interview since he fled to the UK last month, the four-star general told the BBC that Mr Museveni's plans should be opposed.
Gen Sejusa fled after claiming that Mr Museveni was grooming his son Muhoozi Kainerugaba to succeed him.
The government has repeatedly denied any such plan.
Mr Museveni has been in power since 1986, and elections are due in 2016.
The security forces last month raided the offices of two newspapers which published a confidential letter by Gen Sejusa, calling for an investigation into allegations of a plot "to assassinate people who disagree with this so-called family project of holding onto power in perpetuity".
Gen Sejusa told BBC Focus on Africa that Mr Museveni was ruling over a "decadent system" and "playing God" in Uganda.
"Who gave Mr Museveni the right to rule over us forever?" he asked.
Gen Sejusa said Brig Kainerugaba represented a "national project of Mr Museveni to subvert the existing political system in order to perpetuate himself".
"The central issue is a political monarchy - a life presidency and then transiting [to] a political monarchy," he added.
"It is a terribly common African story. There is nothing strange about it."
Pressed to rule out suggestions that he harboured presidential ambitions, Gen Sejusa said: "Why should I? A four-star general without ambitions - he might be in a wrong place."
Government spokesman Ofwono Opondo denied there was any "project" that would lead to Brig Kainerugaba succeeding Mr Museveni.
He told Focus on Africa that Gen Sejusa had never raised his concerns within the government, even though he served in the military high command and was an MP representing the army in parliament.
Mr Opondo denied Gen Sejusa would be arrested if he returned to Uganda.
"He's a free man to return, if he's not a coward," Mr Opondo said.
However, the government would "deal" with Gen Sejusa if he resorted to unconstitutional means to oppose the government, he added.
Gen Sejusa told the BBC the constitution gave people the right to "use all means necessary.... including, by the way, armed struggle" to resist a leader who subverted democracy.