Iran has begun several days of public mourning and funeral processions for its former Supreme Leader, Ayatollah Ali Khamenei, more than four months after he was killed in strikes launched by the US and Israel.

The former Ayatollah's body will lie in state in Tehran's Grand Mosalla from Friday, ahead of his burial in his hometown of Mashhad next Thursday.

Officials have said the funeral could draw up to 20 million mourners from across Iran, according to AFP.

It comes as Iran and the United States observe a fragile ceasefire after signing a preliminary deal to halt their conflict in June.

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

There will be an official funeral ceremony in Tehran on Saturday.

Authorities have ordered public and private offices in Tehran to close from Saturday through Monday, while traffic restrictions will shut down most of the city centre to private vehicles, AFP reported. The airspace over Tehran will be partially closed from Friday and fully closed on Monday.

Following the ceremonies in Tehran, the former supreme leader's body will be taken to the Iraqi holy cities of Najaf and Karbala, prior to his burial in Mashhad.

Representatives from multiple countries are expected to attend the ceremonies, including Pakistan's Prime Minister Shehbaz Sharif.

Khamenei was succeeded by his son, Mojtaba, who has not been seen in public since becoming supreme leader.