Nour is seen by some as a symbol of democracy in Egypt

Supporters of Ayman Nour, leader of the Party of Tomorrow and a possible contender for the presidency, insist charges against him were trumped up.

He was freed along with five others also suspected of forging papers to get the new party registered.

Washington has expressed concern over the treatment of Mr Nour.

The six detainees were freed on bail of 10,000 Egyptian pounds ($1,724) each and the investigation is continuing, Egyptian chief prosecutor Maher Abdel-Wahed told reporters in Cairo.

There was however, he said, "no longer any reason for [Mr Nour's] preventive detention".

Challenger

Conveying Washington's concern, Secretary of State Condoleezza Rice had recently expressed the hope that the issue would be "resolved soon".

Nour's party was only launched in October

Egyptian officials have insisted the case is a legal, not political, matter.

Police had seized Mr Nour, an MP, as he left parliament on 29 January and he was accused of forging all but 14 of more than 2,000 signatures required to form his party.

Some analysts believe Mr Nour, a former journalist, lawyer and publisher, possesses the necessary profile to make substantial inroads against the president.

Mr Mubarak agreed to allow challengers to contest the election after demonstrations by pro-democracy campaigners defying a ban on street protests and amid pressure from the US.

Last week, Mr Nour announced his decision to run for the presidency.