Ayman Nur, leader of the Al-Ghad (Tomorrow) Party, was stripped of parliamentary immunity on Saturday. Police took him off for questioning when he left the parliament building.
The office of the State Security Prosecutor on Sunday ordered him held for four days to investigate charges that many of the documents he submitted with his application for recognition of the party last year were forged.
A judicial source said that during 11 hours of questioning, Nur complained that a police officer had punched him in the face, leaving a bruise under his left eye.
Nur asked to be examined by a doctor, the source added.
Nur's wife, Gamila Ismail, told Reuters that the charges were fabricated to discredit his party, the largest opposition party with six members of parliament.
"The whole issue is political. It's a very mean political conspiracy ... to shake the party's image. It's obvious that the regime so far can't bear any genuine party," she said.
"They are afraid of political competition between the (ruling) National Democratic Party and the Ghad Party in the future"
Group for Democratic Development
Nigad al-Birai, chairman of the human rights organisation Group for Democratic Development, said he had doubts about the validity of the formal charges.
"They are afraid of political competition between the (ruling) National Democratic Party and al-Ghad Party in the future," al-Birai said by telephone from Amman.
"This is only my feeling, but the government has started to put pressure on anyone who is talking seriously about political reform."
Nur would not have needed to forge documents and, as a lawyer, he would have been careful to ensure that all the papers were in order, added al-Birai, who expects to be part of the legal team defending Nur if the case goes to trial.
The judicial source quoted Nur as denying that he had forged any documents and saying the handwriting on some of them was not his. Genuine power of attorney papers could have been replaced with false ones, he added, according to the source.
Pressure for change
Ghad and other opposition parties have been pressing for constitutional change so that the president is elected in direct multi-candidate elections.
Parliament is expected in May to nominate President Husni Mubarak as sole candidate for another six-year term, which would give him 30 years in power. After he is nominated, Egyptians would have a chance to vote yes or no in a referendum in September.
Police sources said 10 students at Minufiya University northwest of Cairo were arrested on Sunday for distributing leaflets hostile to Mubarak.
Mubarak has not faced a serious challenge for previous terms, but this year an informal Enough Movement has sprung up, staging street demonstrations against any extension for Mubarak or any attempt to install his son Jamal as successor.