Nur, leader of al-Ghad (Tomorrow) party and the Egyptian president's main challenger in the September elections, will appeal, Amir Salim, his lawyer, said.
Salim said:"This is a political verdict that will be annulled by the appeal court. This verdict will go into the dustbin of history.
"This is injustice and we are going to take it to the court of cassation."
A court of cassation can overturn the decision of a lower court.
Jamila Ismail, Nur's wife, who has organised daily protests against the trial, led his supporters in chants of "Down with Mubarak, down with the regime".
Nur has been on hunger strike for two weeks in protest at the trial, which he says is a politically motivated attempt to remove him from the political scene.
The forgery charges stem from Nur's application to set up the al-Ghad in 2004, a process which requires hundreds of signatures of endorsement.
"This verdict will go into the dustbin of history"
Ayman Nur's lawyer
During the trial, one of his six co-defendants retracted his confession, telling the court that security officials had coerced him to make a statement accusing Nur of forgery.
The verdicts of the co-defendants were not heard on Saturday as the court erupted in shouts as soon as Judge Abd al-Salam Gumaa delivered Nur's conviction and sentence.
But Egypt's semi-official Middle East News Agency reported that all six were convicted of forgery, with Ismail Zakariya and Ayman Ismail receiving five years in prison, and Lutfi al-Shinawi, Ahmad Abd al-Shafi al-Ghiryani and Mervit Sabir receiving three years each.
Ayman Nur was detained in
January under forgery charges
Faraj Shadid Abd al-Hamid, who was tried in absentia, received 10 years.
The judge gave no explanation of his rulings.
Nur was detained in January and held for six weeks for questioning on allegations that many of the signatures were forgeries. He repeatedly denied it and said the authorities were trying to disrupt his campaign for the presidency.
Hussain Abd al-Ghani, Aljazeera's Egypt correspondent, reported that sources close to Ayman Nur said the whole case was a matter of "political revenge".
Nur won about 8% of the popular vote in the presidential elections, second to Mubarak with 89%, but lost his seat in parliament in November.