Mr Guliyev was held by Ukrainian officials in Simferopol

Rasul Guliyev was due to fly into Baku after nine years in exile in the US, when the Azeri authorities reportedly refused him permission to land.

Upon landing in Simferopol, Mr Guliyev was arrested on an international warrant and taken to a nearby police station, Ukrainian officials said.

Mr Guliyev is wanted by Baku for embezzlement. He rejects the charges.

The leader of the opposition Democratic Party has said he wants to stand in Azerbaijan's parliamentary elections in November.

His spokesmen said that his plane, which had left London earlier in the day, had been refused permission to land in Baku and had to divert to Ukraine.

"We are fighting with the government to get permission to land," Mr Guliyev told Reuters news agency by telephone.

The Azeri authorities insist that they did not prevent him from landing in Baku.

Earlier, they said Mr Guliyev would be arrested upon his arrival in the capital.

Tight security

At least 26 of his supporters - including a former finance minister and a former Baku police chief - were arrested earlier on Monday, said Azerbaijan's Interior Minister, Ramil Usubov.

Security has been stepped up across the capital, Baku

They were planning to stage a protest rally against Mr Guliyev's expected arrest at the airport and to use force against police, Mr Usubov said.

"We will step in and use all means at our disposal to stop any attempts to stage mass unrest in Baku or the regions," the minister added.

Troops reportedly joined traffic police at checkpoints on the road to the airport.

Mr Guliyev, a former parliamentary speaker, is accused of embezzling $117m (�66m) public money in the 1990s. He claims the charges are politically motivated.

Mr Guliyev was once an ally of Haider Aliyev, Azerbaijan's late strongman, the BBC's Natalia Antelava in Baku reports.

It is widely believed that Mr Guliyev fell out of favour because of a conflict with Mr Aliyev's son, Ilham, our correspondent says.