Iran has hanged two convicted drug smugglers inside a Tehran prison, Iran's state-run newspaper said, the latest in a series of executions that has been criticised by rights groups and European governments.

Since authorities launched a clampdown on "immoral behaviour" in July 2007, police have arrested dozens of drug addicts, smugglers, rapists and murderers.

Murder, adultery, rape, armed robbery, apostasy and drug trafficking are all punishable by death under Iran's sharia law, practised since the 1979 Islamic revolution.

"The two men were drug smugglers. They were hanged inside Tehran's Ghezel Hesar prison on Tuesday morning," the Iran newspaper said.

It said a Tehran court last year sentenced one of them to life term in prison but after he tried to smuggle narcotics into the jail, this was upgraded to the death sentence.

"Police found out he was trying to smuggle heroin and opium he got from his friend when returning to the prison from the court," the daily said.

In July, 29 people were hanged in one day inside Tehran's Even prison. In September last year, 21 people were executed in one day in two different places.

Amnesty International in April said Iran executed at least 317 people last year, trailing only China which carried out 470 death sentences.