Hundreds of thousands of people have taken to the streets in two major Iranian cities to mourn Qassem Soleimani, the Quds force general killed in a US drone strike.

The grand funeral procession comes amid soaring tensions with the United States after Friday morning's attack.

Iranian supreme leader Ayatollah Ali Khamenei has promised a "harsh revenge" for the killing, prompting President Donald Trump to threaten a strike on 52 sites in Iran if it retaliates by attacking US citizens.

Thousands mourned the 62-year-old in the Iraqi capital Baghdad, where he was targeted, before Soleimani's body and those of other Quds figures killed was flown to the southwestern Iranian city of Ahvaz.

The flag-draped coffins were met there by an honour guard.

On Sunday morning, thousands dressed in black chanted "Death to America" and beat their chests in Ahvaz.

In a rare honour, Ayatollah Khamenei is himself expected to lead the mourning in the capital city and pray over Soleimani's remains at Tehran University on Monday.

His remains will then be taken to the holy city of Qom, ahead of a funeral on Tuesday in Kerman, his hometown.