British embassy employee Hossein Rassam is accused of spying
European nations have strongly criticised the latest trials in Iran over the unrest that followed the disputed presidential poll.
Iranian workers at the French and British embassies and a French national were among dozens of detainees to appear in court in Tehran.
The EU presidency said action against any EU national or embassy would be seen as an act against the whole bloc.
The UK and France also spoke out against the trial.
This is the second group trial of those accused of taking part in the mass protests that erupted after the 12 June election.
Last week more than 100 people, including leading reformers, appeared in court in Tehran on charges including conspiracy.
'Spying for foreigners'
The group that appeared in court in Tehran on Saturday included reformist lawmakers and journalists, all wearing grey prison clothes.
They are accused of crimes including rioting, spying and plotting to overthrow the government.
Hossein Rassam, the most senior Iranian employee at the British embassy, stands accused of "spying for foreigners".
Prosecutors accuse him of monitoring the riots on the ground along with two UK diplomats who have since been expelled.
An Iranian employee of the French embassy, Nazak Afshar, was also in the dock, as was 24 year-old French teacher Clotilde Reiss.
She has been charged with acting against national security by joining protests and gathering information.
Iranian state media reported that all three apologised to the court for their actions.
European nations reacted with anger. "This is obviously a show trial directed against the EU," Swedish Foreign Minister Carl Bildt told Reuters news agency.
The Swedish EU presidency said the trial was an act against the entire bloc.
"Actions against one EU country, citizen or embassy staff is considered an action against all of the EU, and will be treated accordingly," it said in a statement.
British Foreign Minister David Miliband criticised what he called "unjustified charges" against a member of embassy staff "going about his legitimate duties".
France's Foreign Ministry described charges against Ms Reiss as "devoid of foundation". Ones against Nazak Afshar were "non-existent", it said.
BBC Tehran correspondent Jon Leyne says it is now up to the EU whether they can agree new diplomatic measures, such as the withdrawal of ambassadors, or perhaps new financial sanctions.
At least 30 people were killed in the violence that followed the 12 June poll.
Official results gave victory to incumbent President Mahmoud Ahmadinejad, but the opposition says the poll was rigged.
The Iranian government has accused Western nations of fuelling the violence.
Human rights groups say the Iranian government has detained hundreds of reformers, journalists and opposition supporters since the poll.
Mr Ahmadinejad was sworn in earlier this week, but a number of top Iranian officials did not attend.