Nearly four months after his death, Ayatollah Ali Khamenei's burial is taking place as Iran and the US observe a fragile ceasefire.

Waving blood-red flags seen as a call for vengeance and justice under Shia Islam, mourners thronged the Grand Mosalla religious complex.

The mourners, some in tears, made their way through strict security towards the courtyard where Khamenei's coffin was placed for people to pay their respects.

Iran's government is preparing a multiday funeral ceremony for late Supreme Leader Ali Khamenei, who died in US and Israeli strikes in February. But many Iranians are not mourning his loss, and are instead sharing complex reactions to the death of a man who led an oppressive theocratic regime.

Before dawn, hundreds of mourners gathered in Tehran as Iran began days of funeral ceremonies for Supreme Leader Ayatollah Ali Khamenei, who was killed months earlier in a joint US-Israeli airstrike.