At least 2,500 people have been killed in Iran since protests broke out at the end of December, the Norway-based human rights organization Hengaw said on Tuesday.

The group noted that its verification team is working daily to document deaths linked to the state crackdown on mass protests.

On Tuesday, Iran Human Rights (IHRNGO), which is also registered in Norway, put the number of demonstrators killed at at least 734, but warned the real toll could be significantly higher, even in the thousands.

"Due to the complete internet shutdown since Thursday night and the severe restrictions on access to information, independent verification of these figures is currently extremely difficult," the IHRNGO report said.

Iranians have been demonstrating for more than two weeks against the authoritarian rule of the Islamic Republic. The protests, triggered by an economic crisis, have meanwhile taken on a nationwide dimension.

Iran has defended its actions against the unrest, justifying the use of force by saying that those taking part were "terrorists" or "rioters."