Iranian authorities have taken senior opposition leaders Mir Hossein Mousavi and Mahdi Karroubi to a prison in Tehran, a reformist website reported Monday.

Kaleme.com said Mousavi, his wife Zahra Rahnavard, as well as Karroubi and his wife, Fatemeh Karroubi, were transferred to Heshmatieh prison in the Iranian capital. The men had been under house arrest for more than two weeks. The report said it was not immediately clear when they were transferred to the prison.

The semiofficial Fars news agency, which is close to the powerful Revolutionary Guard, denied that Mousavi and Karroubi had been jailed. Fars quoted an unnamed judiciary official as saying the two opposition leaders were still under house arrest and have not been allowed to leave their homes or contact the outside world.

Calls placed by The Associated Press to judiciary officials went unanswered.

But the imprisonment of Mousavi and Karroubi marks a major escalation of Iran's political crisis amid defiant calls from the opposition to stand up to the ruling system.

Activists and opposition members have demanded Mousavi and Karroubi be released, vowing to stage demonstrations every Tuesday until they are freed.