After a campaign online and through SMS texts to sign up for the human chains nationwide, officials claimed that more than 14 million people had joined.

It was not immediately possible to verify that figure or how many people were taking part, although initial images showed dozens involved at each location.

The state-run IRNA news agency showed people forming a human chain "to support power plants" in the southern city of Bushehr, which is home to Iran's nuclear power plant.

State television and the Mehr news agency showed dozens of people outside the main power station in the northern city of Tabriz, as well as at a plant in the northeastern city of Mashhad

With US-Israeli strikes also targeting bridges, people gathered on the main span over a river in the western city of Ahvaz, Mehr reported.

While the more than five-week war between the Islamic republic and the United States and Israel has seen a whole echelon of the Iranian leadership killed, strikes on energy plants would mark a major escalation.

Iran parliament speaker Mohammad Bagher Ghalibaf, seen by some as the country's de-facto number one after the killing of supreme leader Ali Khamenei at the start of the war, published a screenshot of what was purportedly the registration system for the human chains.

Saying he had registered his own name, he added: "Mohammad Bagher Ghalibaf is ready to sacrifice his life for Iran."

The Persian word for self-sacrifice -- janfada -- became a trending hashtag on social media.

"More than 14 million proud Iranians have so far registered to sacrifice their lives to defend Iran. I too have been, am, and will remain ready to give my life for Iran," President Masoud Pezehskian wrote on X.

US President Trump earlier warned "a whole civilization will die" in Iran if the country did not heed his midnight deadline to open the Strait of Hormuz.