The French and British embassy employees and a French scholar were among 100 defendants who appeared in Tehran's Revolutionary Court as a mass trial of Iranians and others resumed Saturday.
The EU presidency, currently held by Sweden, said Iran will be held responsible.
"The presidency reiterates that actions against one EU country -- citizen or embassy staff -- is considered an action against all of EU, and will be treated accordingly," the European Union said in a statement. "The EU will closely follow the trial and demand that the persons will be released promptly."
Britain's Foreign Office also denounced the proceedings, calling them an "outrage."
Among those named in court Saturday were Clotilde Reiss, a French academic working in Iran, and Nazak Afshar, an employee of the French embassy in Tehran.
France called for the release of both women, saying the charges against them are unfounded.
"France also stands against the conditions of that [court] appearance," the French foreign ministry said. "The embassy was not informed in advance nor permitted to attend the hearing, in accordance with international rules of consular protection. We regret that Ms. Clotilde Reiss and Ms. Afshar are not being assisted by a lawyer."
The semiofficial Fars News Agency said Reiss admitted to her crimes in court Saturday and asked for clemency.
"I shouldn't have participated in the illegal demonstration and shouldn't have sent the pictures, I am regretful," Fars quoted her as saying. "I apologize to the Iranian people and court and I hope the people and the court forgive me."
Human rights groups and Iran's opposition leaders have accused the government of coercing such confessions.
Please continue to pray for these defendants and for the others on trial for peacefully protesting against an oppressive tyranny, and for true justice to triumph.
Iran to Put British Embassy Staff on Trial