Mr Khatami said that last year's presidential election was rigged
Iran's former president has been barred from leaving the country, reports say.
Mohammad 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.
Mr Khatami has been under steady pressure from the Iranian government following the disputed presidential election in June last year.
The Iranian opposition, including Mr Khatami and the defeated presidential candidate Mir Hossein Mousavi, said the election was rigged.
Thousands of opposition supporters took to the streets after it was announced that the incumbent, President Mahmoud Ahmadinejad, had won by a landslide. More than 80 people are believed to have died in the clashes with the security forces.
The BBC's Iran correspondent Jon Leyne, reporting from London, says that since the election, President Ahmadinejad's government has put severe restrictions on the opposition leadership.
He says that when they have tried to attend demonstrations they have faced intimidation from members of the government's Basij militia, and many of their senior aides are in prison.
But so far the government has been wary of going further and arresting the leaders themselves, our correspondent adds.