Online footage shared by Iranian media showed crowds of men and women dressed in black, some carrying Iranian flags, streaming to an open space outside the Grand Mosque of Imam Khomeini in the capital.

This week's Friday prayers were also the first since the killing of Iran's supreme leader Ayatollah Ali Khamenei in the wave of US and Israeli strikes that triggered the Middle East war.

In the background of one video, a man speaking through a loudspeaker mourned the late supreme leader.

"We bear witness that he was the embodiment of piety and guardianship in our time," he said as some worshippers seated on prayer rugs wept.

Recitations of Quranic verses blared from loudspeakers later on at the gathering, which took place amid a heavy security deployment to the area.

Footage from other cities across Iran, including Ilam and Borujerd in the west and Zahedan in the southeast, showed similar scenes, with many worshippers holding Iranian flags.

The war began on Saturday with US and Israeli strikes on Iran which responded by attacking US bases in Gulf countries and mainland Israel with drone and missile strikes.

Iran's health ministry said nearly 1,000 people had been killed in the US and Israeli strikes which hit military sites as well as residential areas and other infrastructure.

AFP could not independently verify the toll.

Iranian attacks killed at least 10 people in Israel according to first responders there, while the US military has reported the deaths of six of its personnel since the war erupted.