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Charles Massi was apparently arrested in Chad in December and taken to CAR, where he died in custody in January.

His family have alleged he was tortured to death, but the government has denied the claims.

Former President Ange-Felix Patasse said the government must "shed light on this alarming case".

In a statement quoted by the AFP news agency, Mr Patasse said his call for an investigation did not mean he endorsed the rebels' cause.

He said he wanted to ensure the nation returned to peace before an election due in April.

Cause celebre

The BBC's Chris Simpson in the CAR capital Bangui says the government has attempted to dismiss Massi as a marginal figure who has taken no part in peace talks.

But in death, Massi has become something of a cause celebre among the opposition and has put the government in an awkward position, says our correspondent.

Massi was a senior political figure in the CAR and had two spells as a cabinet minister - including under Mr Patasse.

But in his mid-50s he turned to the rebel movement, surprising many of his friends and colleagues, our correspondent says.

His group, known as the Convention of Patriots for Justice and Peace (CPJP), had been blamed for several killings.

Government troops were also accused of rights abuses in the northern town of Sokumba last year, where they allegedly killed a number of civilians in an attempt to suppress the rebels.

In a statement last week, former colonial power France said it wanted clarity on Massi's death, arguing that nothing would justify his mistreatment.