Large crowds of black-clad mourners have gathered outside Tehran's main mosque to pay their respects to Iran's former supreme leader on the first day of his funeral comemorations.

Ayatollah Ali Khamenei's body is currently lying in state at the Grand Mosalla, ahead of his eventual burial in his hometown of Mashhad next Thursday.

Authorities expect 15-20 million people to attend the ceremonies across Iran and Iraq over the coming days, which take place more than four months after Khamenei was killed in US and Israeli strikes.

Masses of supporters of the Islamic regime turned out on Saturday morning, reportedly chanting slogans against the US and in support of revenge for the ayatollah's killing.

"We came [to the funeral] because we promised the supreme leader we would stand by him to the very end," 37-year-old professor Reza told news agency AFP in the Grand Mosalla's courtyard.

"For a long time, we shouted that we would sacrifice our lives for the leader, but it was he who sacrificed himself for us."

Arash Rahimi, 40, told Reuters: "Everyone here has come to avenge the blood of their supreme leader.

"As our leader has said, we have a blood feud with the United States. Our relations with the United States will never be good."

Khamenei was killed during joint Israeli and US strikes on Iran in late February, in what quickly spiralled into a wider regional conflict.

US President Donald Trump said Iran's government was "dying to settle" a peace deal to end the war after a preliminary agreement was struck between the two, adding on Friday: "We gave them [Iran] a week off for a funeral because we're nice."

Much of central Tehran will be locked down over the weekend as the funeral ceremony gets under way. It is expected to be the largest funeral ever in terms of attendees as a proportion of the country's population.

Khamenei's body will lie in the Grand Mosalla for three days, alongside the remains of family members who were also killed in the air strikes.

There will then be three further days of events outside of the capital.

On Tuesday, his body will be moved to Qom, to the south of Tehran, where a senior Shia cleric will lead funeral prayers at Jamkaran, one of Iran's most prominent and symbolic religious sites.

It will then travel to Najaf in Iraq on Wednesday. Following a procession at the shrine of Imam Ali, Shia Islam's first imam, ceremonies will continue in Karbala before the body returns to Iran.

Then on Thursday, Khamenei will be buried at the Imam Reza Shrine, the mausoleum of Shia Islam's eighth imam and Iran's most important pilgrimage site, in Mashhad.

Ceremonies beyond the six-day procession will continue across the country for the following 40 days, with commemorative events planned until the first anniversary of Khamenei's burial.

Khamenei was succeeded as Iran's supreme leader by his son, Mojtaba, who has not been seen in public since assuming the role, prompting speculation about his health.

Whether he will be seen at the funeral remains a key question surrounding the carefully choreographed ceremony.