Iranian students say they have been banned from holding a demonstration to support reformist candidates barred from this month's election.
The students had wanted to protest outside their university
The student news agency, ISNA, quoted the Tehran governor's office as saying no authorisation had been given for a rally planned in the city on Wednesday.
Student protests last year sparked a national security crackdown.
Iran has been embroiled in a three-week row between conservatives and reformists over the 20 February poll.
The Tehran University branches of the Islamic Association said they had requested permission to stage a demonstration outside the main campus.
But Ali Taala, director of political and security affairs for the city, was quoted as telling the students: "No gathering will be allowed outside the entrance of Tehran University."
Correspondents say students are often seen as a driving force behind the reformist movement.
Last week, the main pro-democracy student movement, the Office to Consolidate Unity, OCU, called for a national boycott of the elections from which thousands of candidates have been banned by the conservative Guardians Council.
The students also called for a referendum on the country's political future.
But Foreign Minister Kamal Kharrazi said on Tuesday that he hopes the controversy will be resolved to allow free and fair elections.
Speaking during a visit to Ukraine, Mr Kharrazi said Iran was following a path of democratic change.