Former president Mohammad Khatami has reportedly been barred from leaving Iran.
Mr Khatami was expected to attend a nuclear disarmament conference in Japan, but his aides say he was banned from travelling.
Organisers of the meeting in Hiroshima have confirmed that he cancelled his appearance at the last minute.
The BBC reports Mr Khatami has been under steady pressure from the Iranian government since the disputed presidential election in June last year.
Mr Khatami and defeated presidential candidate Mir Hossein Mousavi both claimed the election - won by incumbent President Mahmoud Ahmadinejad in a landslide - was rigged. More than 80 people are believed to have died in subsequent clashes with security forces.