Mr Ouatarra was held under police guard at his home in Abidjan on Thursday, and questioned over allegations that he had falsified papers connected to his citizenship.

The investigation proceeded amid violent demonstrations in the capital by Mr Ouatarra's supporters, in which police have now admitted that one person was killed.

The government has also imposed a three-day ban on broadcasts by an opposition radio station.

If Mr Ouatarra is not Ivorian, he would be ineligible to contest presidential elections.

But the opposition leader, whose career includes stints as Ivorian prime minister and as a top International Monetary Fund official, insists he is Ivorian.

Death mystery

The circumstances of the death during the demonstrations are not clear.

A senior policeman in the district of Abidjan where the corpse was found said his men were not responsible.

A spokesman for Mr Ouatarra's Rally for Republicans party had earlier implied that the security forces were to blame, but he admitted he could not confirm this.

More than 300 of Mr Ouatarra's supporters who were detained following a scuffle outside the opposition leader's house on Wednesday have since been released.

Radio ban

Alasanne Ouatarra was questioned under police guard at his home

The private broadcaster Radio Nostalgia was taken off the air on Friday, after being suspended from transmitting for three days.

The National Media Council imposed the ban, saying the station had failed to "respect a convention with the state".

But managing director Ahmed Bakayoko said the radio station was being "punished for being close to Mr Ouattara".

Radio Nostalgia, Ivory Coast's second most popular station, was earlier suspended for six days in August.

IMF concern

A senior official at the IMF - where Mr Ouattara was formerly a deputy managing director - said he feared for Mr Ouattara's safety while he was being questioned.

"I have, in a personal capacity, expressed to the authorities our great concern about the personal integrity, health and safety of Mr Ouattara," the official told reporters on condition of anonymity.

The official added that developing countries "must be able to demonstrate also their commitment to a fair and proper working of democracy".

Mr Ouattara left the IMF earlier this year to stand as a presidential candidate in Ivory Coast.

Politicised

West Africa correspondent Mark Doyle says the dispute over Mr Ouatarra's nationality has become deeply politicised, in a country which is usually one of the most stable in the region.

Investigations appear to centre on a police theory that Mr Ouatarra's mother came from Burkina Faso, but died when he was a child.

The police apparently believe that Mr Ouatarra may then have claimed an Ivorian woman as his real mother.

This Ivorian woman could also have been married to Alassane Ouatarra's Ivorian father, since polygamy is practised in parts of Ivory Coast.

If it is true, Mr Ouatarra would not be eligible to stand in the elections.