The Egyptian government says the court trying Nour is neutral
Nour, who came second in a presidential poll in September, was first held in January this year accused of falsifying signatures to register his party, Ghad.
Egypt denies Nour's claim that the charges are politically motivated. And the US has voiced concern at the trial.
Hundreds of Nour's supporters at the court shouted slogans against President Hosni Mubarak as the verdict was given.
"Hosni Mubarak's rule is illegal! The trial is illegal!" they chanted.
According to the BBC's Bethany Bell in Cairo, the streets near the court were full of riot police and Ghad party supporters.
Nour has been in hospital after starting a hunger strike earlier this month in protest at his detention.
US concern
His lawyer, Amir Salim, is quoted by the Associated Press news agency as saying the decision against him will "go into the dustbin of history".
"This is a political verdict that will be annulled by the appeal court," he said.
A co-defendant in the trial, Ayman Ismail, had admitted forging documents for Nour - but later withdrew his testimony, saying the confession was forced out of him with threats against his family.
Despite the charges against him, Nour was allowed to compete in presidential polls, where his party finished second to Mr Mubarak's.
He lost his assembly seat to a candidate from the ruling party in November.
The has US earlier said it was watching Nour's trial, which it regards as a test of Cairo's tolerance of dissent.
State department spokesman Adam Ereli said this month that the US was calling on Egypt "to make every effort to ensure that this trial conforms to international standards".
Egyptian opposition leader and former candidate in presidential election Ayman Nour has been sentenced today for 5 years imprisonment, as it was reported by both officials and Nour's lawyer. According to the court, he and his liberal democratic party, Ghad, falsified signatures required to register a political party in January 2005. Both Nour's punishment and his imprisonment have been condemned by the international community.

Nour, who stood as a candidate in the recent presidential election, held on September 7 2005, claims that there was political pressure on the judgement and he'll take it to the court of cassation. The leader's lawyer, Amir Salim, said that the verdict would be annulled by the appeal court, as Associated Press reports.

Groups of Nour's supporters, organized mainly by his wife Gameela Ismail, protested against the decision and the style of government of current president Hosni Mubarak, shouting "Down with Mubarak, Down with the regime."

Ayman Nour was arrested in January, 2005 and then imprisoned. He was released when the presidential campaign started.