Clutching portraits of their hero, the crowd gathered on the outskirts of Tehran University where the supreme leader is to preside over prayers for the slain general, an AFP correspondent reported.

In freezing early morning temperatures, women in black walked alongside men holding up flags bearing the names of Shiite imams as they walked down Enghelab (Revolution) Street towards the university.

"Down with USA," said a placard held up by a young woman wearing a black chadour.

The procession was broadcast live on state television, with screens bearing a black ribbon across the top left hand corner in a rare tribute.

Soleimani, who spearheaded Iran's Middle East operations as commander of the Revolutionary Guards' Quds Force, was killed in a US drone strike Friday near Baghdad airport. He was 62.

The attack was ordered by President Donald Trump, who said the Quds commander had been planning an "imminent" attack on US diplomats and forces in Iraq.

Supreme leader Ayatollah Ali Khamenei vowed "severe revenge" and declared three days of mourning.

- '#Hard_revenge' -

Iran said late Sunday it was further rolling back its commitments to a nuclear deal in tatters since Trump unilaterally withdrew from it in May 2018.

The landmark accord agreed in 2015 with six major powers gave Iran relief from sanctions in exchange for limits on its nuclear programme.

Sunday's announcement was yet another sign of the fallout from Friday's killing of Soleimani.

The general's assassination ratcheted up tensions between arch-enemies Tehran and Washington and sparked fears of a new Middle East war.

In Tehran on Monday, mourners formed a sea of black along Enghelab Street dotted with red Shiite flags and white signs.

One of the placards held aloft by a man read "#hard_revenge".

As they marched down Enghelab Street, a main artery of the Iranian capital, the mourners chanted "Death to America" and "Death to Israel" and waved flags of Iraq and Lebanon among others.

The procession comes after Soleimani's remains returned to Iran on Sunday.

They were paraded through the streets of the southwestern city of Ahvaz before being taken to second city Mashhad, in the country's far northeast.

The Revolutionary Guards said the overwhelming number of mourners in Mashhad forced the cancellation of a ceremony that had been planned in Tehran on Sunday night.