Four erstwhile Kagame allies called on the international community to exert pressure to end impunity for gross human rights violations and help avert conflict which they said is inevitable if he continues to marginalise the Hutu population.
"Rwanda is a one-party authoritarian state, controlled by President Kagame through a small clique of Tutsi military officers and civilian cadres of the (ruling party)," the group said in a 60-page document seen by Reuters on Thursday.
"The consolidation of authoritarian rule enhances prospects for violent conflict instead of consolidating peace and stability," the four former Kagame allies said.
Rwandan government officials did not immediately respond to requests for comment.
The quartet -- a former attorney general, army chief, head of external intelligence and ambassador to the United States -- accused Kagame of wielding absolute control over the judiciary and legislature.
"(The) lack of an independent and impartial judiciary is probably the greatest constraint to the development of democracy in Rwanda," they said.
Kagame is praised for establishing stability, completely rebuilding the country in the aftermath of the slaughter of over 800,000 Tutsi and politically moderate Hutu 16 years ago, and overseeing robust economic growth, albeit from a low base.
Under Kagame gross domestic product per capita has trebled, tax revenues have leapt, and Rwanda is now considered the top global business reformer and least corrupt nation in the region.
But the quartet said Rwanda's transition to democracy since the 1994 genocide has failed and they called for a coalition government which would free political parties and the press, nurture civil society, disband informal security networks and initiate security sector reform.
In recent months, Kagame has fiercely fended off criticism from rights groups who said the run-up to his August landslide re-election was marred by mounting repression and violence against opponents and critics.
The genocide was spawned, in part, by the surge of radical ethnic politics that followed the birth of multi-party democracy in Rwanda in the early 1990s. In the aftermath of the genocide, Kagame says he has prioritised stability and development over "niceties" such as democracy.
"President Kagame muzzles domestic opponents and critics through unapologetic terror," the document said.
The group accused Kagame of being a source of instability in the region, having invaded neighbouring Democratic Republic of Congo twice and supported proxy militias in its mineral-rich eastern borderlands.
They also said Kagame's strident leadership has paralysed decision-making at senior levels of government and that a climate of fear had caused a brain drain of the educated elite.
The group said long-term stability was at risk from emerging divisions within the army as well as from disenfranchised Hutu.
"Purges of political enemies, real and imagined, within the ruling party and government continue unabated. These purges have now been extended to the military," they said, referring to the arrest of several senior army officials earlier this year.
One of the authors, former army chief Faustin Kayumba Nyamwasa, was shot outside his home in South Africa in June, having fled Rwanda in February following charges of terrorism.
(Editing by David Clarke)