Ayman Nur, the sole representative of al-Ghad, or Tomorrow, Party in parliament, was arrested on Saturday after a session that removed his parliamentary immunity.
He denies the forgery accusations.
Five Egyptian rights groups protested against the parliament's move on Monday.
In a joint statement, the organisations expressed their "strong indignation over the stripping of Nur's parliamentary immunity in such a hurried way".
They said the action by the parliament might be "a message to other political parties".
Nur has alleged being mistreated during his detention.
"I was attacked and pushed to the ground... I received blows under the right eye and was hit on the back several times," he was quoted as telling investigators in a statement issued by the Egyptian Organisation for Human Rights.
Human rights violations
That statement said Nur's first interrogation session lasted eight hours non-stop, in violation of international agreements on human rights and the international agreement against torture signed by Egypt.
"I was attacked and pushed to the ground. ... I received blows under the right eye and was hit on the back several times"
"The organisation appeals to the authorities to release Nur soon according to the Egyptian laws and constitution and in respect for the human rights values and principles," it said.
In Washington, Department of State spokesman Richard Boucher expressed concern about Nur's detention and reports of his mistreatment.
He also urged the Egyptian government to take notice of his health condition.
"We note he's a diabetic who needs regular medical attention, and we would hope that the Egyptian government would make sure that he's properly treated and that they would re-examine the issue given that he is an opposition member of parliament," Boucher said.
Forgery accusations denied
State security investigators have accused Nur of forging all but 14 of the more than 2000 signatures he was required to furnish to the committee that licenses political parties. Nur, who got his party approved in October, rejected the forgery charge in a speech to parliament, saying the Political Parties Committee had the original signatures.
Egypt's Husni Mubarak appears
set to seek a fifth term
Nur, 45, appeared in a heavily guarded court on Monday. He told the judge that his arrest would "hinder the democratic process and reform in Egypt".
President Husni Mubarak's ruling National Democratic Party met with opposition parties on Monday for talks on political reform in Egypt.
Academics and opposition newspapers have accused the government of rigging elections. The parliament has been dominated by the ruling party since political parties were revived in the 1970s.
Nur is among those government opponents who have called for the constitution to be amended to allow more than one candidate to run for the presidency.
Egypt holds presidential referendums in which people vote "yes" or "no" for a single candidate, who has been approved by parliament.
Mubarak hinted on Saturday he would stand for a fifth term in office later this year. There have been some protests against his re-election.
The rights groups that protested against the removal of Nur's immunity were the Arab Centre for the Independence of the Judiciary and Legal Profession, the Arab Programme for Human Rights Activities, Hisham Mubarak Legal Centre, the Egyptian Organisation against Torture, and the Land Centre for Human Rights.