Nour says the charges against him are politically motivated

Ayman Ismail said security agents had threatened to harm his family if he did not testify against the politician.

Mr Nour denies forging signatures to register his party and says the charges are politically motivated.

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

If convicted, he would lose his right to take part and could face up to 15 years in prison.

'Concocted'

Mr Ismail had admitted forging documents at Mr Nour's request.

The trial opened in chaos as supporters confronted riot police

But he told the Cairo court that "security people threatened to hurt" his nephews if he did not confess.

"Anybody in my shoes would have done the same. I don't know what will happen to me now," he said.

The politician is being tried along with six other defendants, one of whom is being tried in absentia.

Lawyers say some have confessed to forging signatures on the orders of Mr Nour, who claims he has never met five of his fellow defendants.

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

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

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