Huge groups of mourners dressed in black have assembled outside Tehran's central mosque to pay respects to Iran's late supreme leader at the start of his funeral observances.

The remains of Ayatollah Ali Khamenei lie in state at the Grand Mosalla before final burial in his birthplace of Mashhad next Thursday.

Officials anticipate 15-20 million participants at events across Iran and Iraq in coming days, more than four months after Khamenei died in US and Israeli strikes.

Supporters of the Islamic government appeared in large numbers on Saturday morning and chanted slogans opposing the US while calling for revenge over the ayatollah's death.

"We attended because we vowed to stand with the supreme leader until the end," 37-year-old professor Reza told AFP in the Grand Mosalla courtyard.

"We long declared we would give our lives for the leader, but he gave his for us."

Arash Rahimi, 40, told Reuters that everyone came to avenge the supreme leader's blood.

"Our leader said we have a blood feud with the United States and our relations will never improve," he added.

Khamenei died in joint Israeli and US strikes on Iran in late February that quickly widened into regional conflict.

US President Donald Trump said Iran's government wanted to reach a peace deal after a preliminary agreement, adding on Friday that officials gave Iran time off for the funeral.

Much of central Tehran faces lockdown this weekend during the ceremony, expected to draw the largest crowd relative to population of any funeral in the country.

The body will remain at the Grand Mosalla for three days beside family members also killed in the strikes, followed by three more days of events outside the capital.

On Tuesday the body travels to Qom where a senior Shia cleric will lead prayers at Jamkaran, then to Najaf in Iraq on Wednesday for processions at the shrine of Imam Ali and in Karbala before returning to Iran.

Burial takes place Thursday at the Imam Reza Shrine in Mashhad, Iran's main pilgrimage site, with further commemorations across the country for 40 days and up to the first anniversary.

Khamenei was succeeded by his son Mojtaba, unseen in public since taking office, which has raised questions about his condition and whether he will appear at the funeral.