Iran's Supreme Leader Ayatollah Ali Khamenei meets with people of Qom, in Teheran, Iran, January 9, 2019. Official Khamenei website/Handout via REUTERS

DUBAI (Reuters) - A large number of prisoners in Iran will be pardoned on Thursday in honour of the 40th anniversary of the revolution that toppled the U.S.-backed Shah, Supreme Leader Ayatollah Ali Khamenei said on his Twitter account.

The number of prisoners that will be pardoned by Khamenei was not announced, but Iranian media earlier said around 50,000 prisoners will enjoy “the Islamic clemency”.

The country is holding official celebrations for the anniversary of the 1979 Islamic Revolution that will culminate next Monday in a nationwide rally.

It was not immediately clear whether those who will be pardoned will include some of the hundreds of prisoners that human rights groups say have been jailed for political offences.

Several dual nationals from the United States, Britain, Austria, Canada and France have been detained by Iran’s security forces in past years and have been kept behind bars on charges including espionage and collaborating with hostile governments.

Khamenei has the final say on all state matters and issues pardons or reduces sentences of prisoners on special occasions.