A prosecutor also called on Tuesday for Hajjarian to receive the maximum punishment for charges of acting against national security, the state-run Irna news agency reported.
Al Jazeera's Alireza Ronaghi, reporting from Tehran, said Hajjarian gave the court a written defence apologising for his actions.
"He has called for forgiveness and he has said that he is going to resign from the participation front," Ronaghi said.
"People outside the prison are saying these confessions don't mean anything, that they happened under pressure. So it's really difficult to judge from the content of whatever the confessions are."
Iran began the hearings earlier this month for more than 100 people on trial on charges ranging from rioting to spying and seeking to topple Iran's rulers.
The defendants were among hundreds of protesters, activists and journalists detained in a government crackdown following the unrest, which left at least 20 people dead.
At the first hearing, a number of defendants confessed to organising the violence and said opposition claims that the election was rigged were a pretext for the riots.
The trial has also included televised confessions that rights groups say were likely extracted through pressure.
The June election returned Mahmoud Ahmadinejad, the country's president, to power, but the opposition has disputed the poll, saying it was rigged.
Mir Hossein Mousavi, a reformist presidential candidate in the elections, and Mehdi Karroubi, another opposition candidate, led protests in the wake of the elections, sparking a major crackdown by the police and the detention of hundreds of people.
Reformist leaders and human rights groups have complained that some detainees were abused, tortured and raped.
Senior police and judiciary officials have acknowledged that there has been some mistreatment and have called for those responsible to be punished.
According to Karroubi, a group of people who have been released from jail, are ready to provide testimony that they were sexually abused during their detention.
He said four people are willing to give their account to parliament of being tortured and raped.
Parliament will set up a special commission to look into the rape claims and provide the accusers protection, he said.
Commenting on the trials, Sadegh Zibakalam, a professor of political science at Tehran University, told Al Jazeera: "It appears that this is a continuation of the government's strategy, which is trying to convey the message that the leadership of the reformist movement deliberately led people to believe that the results were false and that there was [manipulation] by Ahmadinejad.
"[But] it's not a question of ballot rigging - it was a deep-rooted opposition by Iranian youth, students, intellectuals, writers against Ahmadinejad's policies.
"Whether he won those elections or not doesn't matter. The fact [is] that he remains president [and] there are still a large number of Iranians who are against his policies.
"The Iranian government is trying to blame the reformists for what happened after the elections and, at the same time, we are observing new evidence in Tehran of serious atrocities that took place during the demonstrations."