Human Rights Watch said Friday that the shadowy arrest of the hero of the "Hotel Rwanda" film amounted to an "enforced disappearance", demanding a full account of how he returned to the country.
Paul Rusesabagina became famous after the movie depicted how the moderate Hutu sheltered hundreds of Tutsi at a luxury hotel during Rwanda's 1994 genocide that left some 800,000, mostly Tutsi, dead.
He had been living in exile in the United States until he suddenly appeared under arrest in Rwanda 10 days ago, accused of murder, terrorism and financing rebels.
His family believes he was kidnapped while on a trip to Dubai, arguing he would never return of his own accord to Rwanda.
"Rwanda has an established track record of using unlawful, cloak-and-dagger methods to target those it perceives to be a threat to the ruling party," said Lewis Mudge, Central Africa director at Human Rights Watch, in a statement.
"The fact that Rwanda did not pursue Rusesabagina through lawful extradition proceedings suggests the authorities do not believe their evidence or fair trial guarantees would stand up to scrutiny before an independent tribunal, and so opted to circumvent the rule of law."
The rights group said the fact Rusesabagina was held for three days before his arrest was announced, means they had committed an "enforced disappearance", an international crime.
President Paul Kagame denied he had been kidnapped, suggesting he had been deceived into returning.
"He got here on the basis of what he believed he wanted to do and he found himself here.
"It is like you are dialling somebody you want to talk to and find that you've called the wrong number. That is how it happened. It was actually flawless from what I heard ..." he said.
In the years after the genocide, Rusesabagina -- a Hutu -- became increasingly critical of Kagame's government, accusing his ruling party of authoritarianism and anti-Hutu sentiment.
He started an opposition group, the Rwandan Movement for Democratic Change (MRCD), which is said to have an armed wing called the National Liberation Front (FLN).
In multiple speeches, Rusesabagina has expressed support for the FLN -- which has carried out armed attacks and is described as a terrorist organisation by Rwanda -- but the extent of his involvement in its actions is unclear.
"The gravity of the charges against Rusesabagina do not give Rwandan authorities free rein to resort to the crime of enforced disappearance and ignore due process and international fair trial standards," said Mudge.
HRW called for Rwanda to immediately grant him access to lawyers of his own choosing and to "urgently provide a complete and corroborated account of how Rusesabagina was apprehended and transferred to Rwanda".