Riot police set up blocks both inside and outside the courthouse

Hundreds of his supporters descended on the courthouse in Cairo and were blocked by rows of riot police both inside and outside the building.

Mr Nour says the charges against him are politically motivated.

He wants to challenge incumbent Hosni Mubarak in the first multi-candidate presidential elections, in September.

Wearing orange, the colour of Mr Nour's party, and shouting slogans such as "Justice for Ayman Nour!", hundreds of supporters arrived at the courthouse in a busy part of Cairo.

Their entry was blocked by security guards as trucks of riot police stood by. The road outside the building was closed off.

Mr Nour says he was himself initially refused entry; it took him more than an hour to take his seat in a caged dock.

"This is the worst beginning of a trial, this is a prison and not a courthouse," the Associated Press news agency quoted him as saying.

Washington concern

Mr Nour denies charges that he forged signatures on his party's registration documents.

He is being tried along with six other defendants, one of whom is being tried in absentia. Some have confessed to forging signatures on the orders of Mr Nour, lawyers said.

Nour says the charges against him are politically motivated

The politician claims he has never met five of his fellow defendants.

"The [confessions] are based on the pressure of the authorities and the security apparatus," said Mohamed Abdel Wahab of Mr Nour's defence team.

Mr Nour, who leads the liberal opposition party al-Ghad (Tomorrow), is seen as one of Egypt's leading pro-democracy figures.

If convicted, he will be disqualified from the presidential race.

But his wife fears the trial - which could take up to six months - could prove so exhausting, it might prevent him from standing anyway.

"He's not going to run - how can a candidate, tell me? What kind of a candidate can do any campaign in this way?" said Gameela Ismail.

Mr Nour was released on bail on 12 March after six weeks in prison. His detention without charge raised concern in Washington.

Last week, he was one of a number of opposition activists who met US Secretary of State Condoleezza Rice.

On her recent visit, Ms Rice criticised Cairo for cracking down on dissenters and called on the authorities to ensure the upcoming election was free and fair.