It was the latest in a string of tough jail terms handed down against prominent critics of hardline President Mahmoud Ahmadinejad over the mass protests that broke out after his disputed re-election.
Nabavi, a former deputy speaker of parliament and a member of the central council of the reformist Organisation of Mujahedeen of the Islamic Revolution, received a five year sentence for "disturbing the country's security" and an extra year for "propaganda against the Islamic republic," lawyer Saleh Nikbakht said.
He vowed to appeal against the verdict handed down late on Wednesday, noting that his client insisted on his innocence throughout the trial.
"Mr Nabavi insisted in his closing defence that he had committed no illegal action," Nikbakht said.
The lawyer said the judgement of the court was entirely "disproportionate" as Nabavi had spent "just 20 minutes" at the June 15 protest that was the basis of the charges "before leaving because of his heart disease."
Arrested on the day of the protest, Nabavi was transferred to hospital early last month for treatment for his heart condition.
Nikbakht said that the court granted his client bail at Wednesday's hearing on a surety of eight billion rials (800,000 dollars) pending a possible appeal, and the prison guards have been removed from his hospital ward.
He added that Nabavi will remain in hospital until the weekend for further surgery before going home.
Under Iranian law, any jail sentence of three months or more is subject to appeal and a convict can be granted bail for the 20-day period allowed for one to be lodged.
The lawyer said that the court has yet to hand down its verdict on a further charge of possession of confidential documents. He said there has also been no word on a charge of blocking traffic.
Iranian authorities arrested around 4,000 protesters, journalists and reformist politicans in a massive crackdown on the June demonstrators. At least 140 have been put on trial, including some of the most prominent.
Five people have been sentenced to death. Others have received stiff jail terms although some of those are on bail pending possible appeals.
Among those are former vice president Mohammad Ali Abtahi, who on Sunday received a six year sentence and was then bailed.
Reformist journalist Mohammad Atrianfar, a member of the moderate conservative Executives of Construction group founded by allies of former president Akbar Hashemi Rafsanjani, was released on bail on Tuesday after being sentenced to six years, according to the ILNA news agency.
Reformist journalist Ahmad Zeidabadi was jailed for five years and ordered to serve the sentence in internal exile in the far northeast, an opposition website reported on Monday.
The court set bail at 350,000 dollars, which Zeidabadi's family was unable to raise.
And another leading reformist, Ali Tajernia, was sentenced to six years in jail plus 74 lashes, the ILNA news agency reported. He was released on bail earlier this month.
Iranian American academic Kian Tajbakhsh has been sentenced to 15 years in prison for spying and remains jailed.
