Hundreds of his supporters swarmed the court building on Tuesday denouncing the trial as a government attempt to eliminate him as a rival.
The prosecution of Ayman Nour on forgery charges has drawn the attention on the United States as Washington presses its closest Middle East ally to allow greater democratic change.
His detention for more than a month earlier this year sparked tensions between Egypt and the United States, and US Secretary of State Condoleezza Rice met Nour among a group of pro-reform politicians during a visit to Cairo last week.
Sitting in the courtroom's caged dock wearing a dark suit and tie, Nour said he was innocent of forging signatures to get his opposition al-Ghad party registered last year.
The session then crumbled into arguments between Nour's lawyers and the lawyers of six other co-defendants, two of whom told the court on Tuesday they forged signatures on Nour's orders.
Nour says he has never met five of the co-defendants and accuses the government of framing him.
Judge takes control
As the lawyers shouted back and forth, chief judge Abd al-Salaam Gomaa angrily interjected: "Is it me who is running the session or is it you?"
He then declared a recess and walked out.
US embassy officials were attending the trial as observers.
"A crime has been fabricated, and a situation by which the authorities can benefit has been created either to pressure me, or to try to obstruct my election campaign or to tarnish my image before the voters"
Egypt's opposition presidential candidate
If convicted, Nour would lose his right to run for office and could face a prison term of up to 15 years.
"A crime has been fabricated, and a situation by which the authorities can benefit has been created either to pressure me, or to try to obstruct my election campaign or to tarnish my image before the voters," he said on the evening before the trial.
Nour is the most prominent figure to announce his intention to run for president in September elections, the first ever to be opened to competitors to President Hosni Mubarak.
Though he has not yet formally announced he is in the race, Mubarak is widely expected to run for a fifth term in office.
Mubarak's government has touted the open election as a major democratic reform.
Pro-democracy activists, however, have cited the Nour trial as one example of how the government will not allow a fair competition, saying the reform will not change Mubarak's long-unquestioned hold on power.Hussain Abd al-Ghani, director of Aljazeera's office in Egypt, said Nour has submitted an official complaint to the chief judge, saying that they were barred from entering the courthouse.
Hundreds of Nour supporters mobbed the steps of the Cairo courthouse and nearly lifted their leader off his feet when he arrived for Tuesday's session.
"Justice for Ayman Nour," they chanted, waving flags in the orange colours of the al-Ghad (Tommorow) party. "No to fabrication" of charges, some banners read.
More than 1000 riot police mounted cordons around the court, and amid the crush of supporters and police it took Nour more than an hour to get inside.
Security guards initially barred him from entering with his supporters, but eventually relented.
"This is the worst beginning of a trial, this is a prison and not a courthouse," he said in the court, complaining of the security's initial refusal to let him in.
Barred from courthouse
Jamila Ismail, Nour's wife, who is also head of the party's media and information department, told Aljazeera: "All these people, Nour's attorneys, foreign and Arab reporters have been prevented from entering the courthouse.
Nour's wife, Jamila Ismail, said
the trial is a poltical one
"We have arrived here expecting a fair trial, but Nour, the main defendant, could not even enter the courthouse," she added.
While 500 Nour supporters were allowed into the building nearly 1000 more continued to demonstrate outside.
Pointing at people gathering in front of the courthouse, Ismail told Aljazeera: "All these people have come here to present their genuine support and to prove that no signatures were forged.
"The case is political and not a criminal one."
When asked about any political settlement between al-Ghad and the Egyptian government, Ismail said: "We have no political settlement with the government. We have been offered some political settlements but we have rejected them.
"Al-Ghad party has been attacked by the law and media," she said.