A Rwandan priest-turned-politician said Wednesday he had been blocked from boarding a plane to his country, where he planned to register his opposition party and run for president.

The return of former priest Thomas Nahimana after 11 years in exile in France sparked fierce debate in Rwanda where pro-government media accuse him of preaching ethnic hatred and denying the 1994 Tutsi genocide.

"To our surprise, as we were preparing to board, agents from Kenya Airways came and forbade us from boarding this plane and returning to Rwanda," Nahimana said in a statement, vowing to remain in transit in Nairobi until he was allowed to do so.

The Rwandan Catholic Church in 2011 asked France to stop Nahimana from publishing material which it claimed denies the 1994 massacre in which mostly members of the Hutu ethnic group killed some 800,000 people in a 100-day genocidal spree.

He and another priest Fortunatus Rudakemwa in Italy, both of the majority Hutu ethnicity, launched a website attacking the government of President Paul Kagame and honouring the memory of Hutus killed by his Rwandan Patriotic Front, the former rebel group now in power.

Kagame has ruled the small east African country since his forces stopped the genocide against his Tutsi minority, and he is seeking re-election in 2017 after the country's constitution was changed last year.

While he is regularly praised for the stability and economic performance of his small nation, Kagame is also often criticised by rights groups for lack of political freedoms and freedom of expression.

All recognised parties generally support the policy decisions made by the ruling Rwandan Patriotic Front (RPF) -- with the exception of the small Democratic Green Party which was the only one to object to the 2015 constitution changes.

In 2010 Rwandan opposition figure Victoire Ingabire was arrested shortly after her return from exile to Rwanda to run for election.

After laying flowers at the genocide memorial in Kigali, Ingabire called for the perpetrators of crimes against the Hutu majority also to be pursued and prosecuted.

She was convicted in 2013 for her comments, viewed as "genocide denial", and is currently serving a 15-year prison sentence.