Mojtaba Khamenei, 56, was named the Islamic republic's new leader on Monday, 10 days into the war that began when the United States and Israel launched strikes on Iran and has since ensnared the Middle East.

Ali Khamenei, who held the Islamic republic's top post for decades, was killed in the initial strikes of the US-Israeli campaign on February 28, along with his wife and daughter-in-law.

"Death to America, Death to Israel," and "God is Great," chanted the defiant crowds that gathered in Tehran's emblematic Enghelab Square on Monday, flanked by a heavy security presence with armoured vehicles and personnel stationed on the rooftops of surrounding buildings.

The new supreme leader, a discreet figure who rarely appeared in public or spoke at official events, has yet to address the nation since his appointment.

His election comes amid Israeli threats to target Iran's new supreme leader, while US President Donald Trump warned that the new ruler, if chosen without his approval, "isn't going to last long".

Trump had insisted he wanted to be involved in the appointment of Iran's new leader and dismissed the younger Khamenei as a "lightweight" and an "unacceptable" option.

- 'Complete obedience' -

Iran's Assembly of Experts, the body tasked with selecting the supreme leader, nonetheless appeared unfazed by the threats.

"The path of the martyred Imam Khamenei will carry on under the name of Khamenei," said Hosseinali Eshkevari, a member of the body, ahead of the official announcement of the new leader's name.

Another member, Mohsen Heydari, said the late Ali Khamenei had recommended the selection of the candidate who is "hated by the enemy".

It was not clear how the vote for Mojtaba Khamenei was held, but Hossein Redaei, another member of the body, suggested that it took place without an in-person meeting given the wartime conditions.

At the gathering in Tehran, one woman formed a "T" with her hands -- a defiant gesture to signal that enemies would leave the battlefield dead.

Others, including women dressed in black chadors, some with their children in strollers, raised their fists and chanted the name of Mojtaba Khamenei.

Many clerics in white and black turbans attended the gathering as Iran's national anthem blared from loudspeakers across the square.

Shortly after his election, the Islamic Revolutionary Guard Corps, the ideological arm of Iran's military, pledged allegiance to the new leader, saying they were "ready for complete obedience".

Other key institutions, including the foreign ministry and the judiciary, also swore fealty.

"Your selection by the Assembly of Experts as the leader of the Islamic Republic system has caused the hostile and warmongering enemies to despair," said Iran's security chief Ali Larijani.