Reiss, a 24-year-old student, was detained at the airport in Tehran over a number of photographs she took at one of the demonstrations in Isfahan, which she then emailed to a friend.
"My motive to take part in the gatherings was a personal one," the state-run IRNA news agency quoted her as saying in court.
"But I accept that it was a mistake and I should not have gone to these gatherings."
Local staff members from the British and French embassies were also appearing before the court.
On Saturday, Abdolreza Mohabati, the prosecutor, said: "Some British diplomats took part in illegal Tehran gatherings.
"The political section of the British embassy was collecting information about officials, the Revolutionary Guards, Basij militia ... it formed a working group to monitor news and the local staffers and diplomats made provincial trips. The embassy also sent local staffers to scenes of unrest."
Mohabati also accused Washington of running an "exchange programme where members of the Iranian elite were sent to the United States for higher education."
"The programme aimed at changing views in Iranian society ... infiltrate the social layers, weaken Iran's government to eventually topple the regime," he said.
Mohabati also accused Voice of America radio and social-networking sites such as Twitter and Facebook of playing a role in spreading the unrest.
The Iranian government has repeatedly accused Western governments of interfering in the aftermath of the election and detained nine local staff from the British embassy during the crackdown.
The British government confirmed that Hossein Rassam, one of those arrested after the protests, was among those seen in the court.
"This is completely unacceptable and directly contradicts assurances we had been given repeatedly by senior Iranian officials," a foreign office spokeswoman said.
"We deplore these trials and the so-called confessions of prisoners who have been denied their basic human rights."
IRNA reported that Rassam had asked the court to show clemency.
"Hossein Rassam expressed regret over his past activities and mistakes and the clear violations he had committed," he said.
"He apologised to the Iranian nation and asked the court for forgiveness."
He also said that the British embassy shared information it had collected with Washington "because the American government lacks facilities to survey Iran events and because of the close relations between Washington and London".
Others on trial on Saturday included key supporters of Mir Hossein Mousavi, who finished second to Ahmdinejad in the presidential election.
They include Ali Tajernia, a former MP, Javad Emaam, the head of Mousavi's campaign office in Tehran, and Shahabeddin Tabatabaee, head of Mousavi's young supporters national committee.
The session was to be held on Thursday but was pushed back to Saturday after some of the lawyers of the defendants had asked for more time to study the charges against their clients.
At the first hearing held last week, a number of defendants confessed to organising the violence and said that the opposition claims that the election was rigged were a pretext for the riots.
Mohammad Ali Abtahi, a former vice-president, Behzad Nabavi, a former industry minister and deputy speaker of parliament, and Abdollah Ramazanzadeh, a former government spokesman, are among the 100 defendants.
Opposition supporters have continued to protest against the election result, but Ahmadinejad was formally sworn in as president on Wednesday with the backing of Ayatollah Ali Khamenei, the supreme leader.
Meanwhile, Amnesty International said there had been an "alarming spike" in executions in Iran since the June 12 vote.
"In just over 50 days, we recorded no less that 115 executions, that is an average of more than two each day," Irene Khan, secretary-general of the human rights organisation, said in a statement.
"This represents a significant increase, even compared to the appallingly high rate of executions that has been so long a feature of the human rights scene in Iran."
It said that most of those executed were said to have been convicted of drug smuggling or related offences. There was no indication that any of those put to death had any connection with the election violence.
In 2008, Iran executed 246 people, second only to China. Tehran says the death penalty is a necessary to maintain public security and is only applied after exhaustive judicial proceedings.