Mr Aminzadeh (left), a former deputy foreign minister, went on trial in August
An Iranian opposition leader has been jailed for six years for his role in the unrest following June's disputed presidential election, reports say.
Mohsen Aminzadeh was convicted of organising protests, disturbing security and spreading propaganda against the system, his lawyer said.
He was a prominent supporter of the defeated candidate Mir Hossein Mousavi.
Meanwhile, opposition websites say nine journalists have been detained in the past two days, taking the total to 55.
There has so far been no confirmation from the Iranian government.
Correspondents say the authorities are tense ahead of the 31st anniversary of the Islamic Revolution on Thursday.
Opposition supporters have called for widespread anti-government protests to coincide with official rallies. The police have meanwhile warned that opposition demonstrations will be firmly confronted.
In December, eight people were killed in clashes at demonstrations on Ashura, one of the holiest days in the Shia Muslim calendar.
In a joint statement, the US and the European Union expressed concern about "continuing human rights violations in Iran" since the election and about the possibility of further violence as the revolution's anniversary is marked.
"The large scale detentions and mass trials, the threatened execution of protesters, the intimidation of family members of those detained and the continuing denial to its citizens of the right to peaceful expression are contrary to human rights norms," the statement said.
Speaking to the semi-official Isna news agency on Monday, Mr Aminzadeh's lawyer said his client had been sentenced to six years in prison by a branch of the Revolutionary Court in Tehran.
"He was charged with organising gatherings and disturbing the country's security, as well as spreading propaganda against the system by giving interviews to foreign channels," Abbas Shiri was quoted as saying.
"Rejecting the charges, I will submit the appeal within the legal period," he added.
Mr Aminzadeh, a former deputy foreign minister under President Mohammed Khatami and a leading member of the Islamic Iran Participation Front, was head of the opposition coalition's headquarters, according to Isna.
He was arrested a week after the 12 June presidential election, the result of which prompted millions to take to the streets demanding a re-run in the largest demonstrations in Iran since 1979.
RECENT UNREST IN IRAN
19 Dec: Influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri dies aged 87
21 Dec: Tens of thousands attend his funeral in Qom; reports of clashes between opposition supporters and security forces
22 Dec: Further confrontations reported in Qom
23 Dec: More clashes reported in city of Isfahan as memorial is held
24 Dec: Iran reportedly bans further memorial services for Montazeri except in his birthplace and Qom
26 Dec: Clashes reported in central and northern Tehran
27 Dec: At least eight dead following anti-government protests in Tehran; 300 reported arrested
Mr Mousavi's Green Movement said the poll had been rigged to ensure the re-election of President Mahmoud Ahmadinejad, a charge the government denied.
At least 30 protesters have been killed in clashes since the election, although the opposition says more than 70 have died.
More than 80 people have been jailed for up to 15 years - including former government spokesman Abdullah Ramezanzadeh, former vice-president Mohammad Ali Abtahi and former deputy economy minister Mohsen Safaie Farahani. Some 200 activists remain in detention.
Last month, two alleged members of a banned monarchist group were executed. Human rights groups condemned the hangings, accusing Iran of staging show trials and of seeking to intimidate the opposition.