July 6 (Reuters) - Crowds of Iranians marched through the streets of Tehran on Monday in a funeral procession for slain leader Ayatollah Ali Khamenei, the biggest day yet in a week of massive memorial ceremonies demonstrating the grip of surviving clerical leaders.

Drone footage on state television showed many tens of thousands of people crammed into a boulevard in central Tehran.

The coffins of the slain leader and four of his family members were driven in a large truck through the streets, while fire hoses sprayed water from above to keep the marchers cool.

As they passed under a bridge mourners hurled stones at a billboard hung from above showing U.S. President Donald Trump with a bullet aimed at his head.

"The U.S. killed our father," it read. "We won't let you go!"

The crowds waved Iranian flags and red banners with a slogan calling out to the "avengers of Khamenei", adapting a phrase at the heart of Shi'ite Islam since the grandson of the Prophet Muhammad was killed in battle in the seventh century.

SONS PRAY FOR KHAMENEI BUT NO SIGN OF MOJTABA

On Sunday, three sons of the slain leader prayed beside his coffin at a huge Tehran prayer hall.

But Mojtaba Khamenei, the son who succeeded him as Iran's supreme leader, did not make an appearance. Believed to have been disfigured by wounds in the attack that killed his father, the younger Khamenei has yet to be seen in public since the war began on February 28 with Israeli and U.S. airstrikes on Iran.

The mourning ceremonies began on Friday when the coffins of the elder Khamenei, one of his daughters and her 14-month-old child, one of his sons-in-law, and the wife of Mojtaba lay in state for Iranian officials and foreign dignitaries. Further large-scale ceremonies were held outdoors on Saturday and Sunday before Monday's huge funeral procession.

Later this week the body will be taken for further processions in Iran's Shi'ite seminary city of Qom and two Shi'ite shrine cities in neighbouring Iraq, before returning to Iran for burial within a medieval shrine complex in Mashhad.

The war has ended with a preliminary peace agreement reached last month that left Iran's clerical leadership in power and claiming victory, with newfound clout from exerting control over global energy supplies through the Strait of Hormuz.

Trump has also claimed victory, although the aims he gave at the outset - destroying Iran's nuclear and missile capabilities, ending its ability to attack neighbours and creating conditions for Iranians to topple their leaders - have yet to be achieved.

Trump said over the weekend that peace talks with Iran had been delayed for a week by the funeral ceremonies.

Israel's defence minister, Israel Katz, said on Monday that the elder Khamenei had been killed because he led a programme to destroy Israel.

"Any Iranian leader who will again try to pursue plans to destroy Israel will be killed as well," Katz said.

(Writing by Peter GraffEditing by Gareth Jones)