Tehran, Iran - The remains of Qassem Soleimani, head of the country's elite Quds Force killed in a US air strike, arrived in Tehran on Monday after a day of funeral processions in the cities of Ahvaz and Mashhad, which saw hundreds of thousands - if not millions - of people pour onto the streets in a burst of grief and anger.

The reception to Soleimani's assassination on Friday has already been compared with the burial of Iran's first Supreme Leader Ayatollah Khomeini, who died of natural causes and was buried in Tehran in 1989. Millions of people took to the streets that day, in what was then described as the biggest funeral procession in Iran's modern history.

Iranian leaders have pledged "severe revenge" in response to Soleimani's killing, which has triggered a dramatic escalation of tensions in the Middle East.

Abbas Aslani, of the Center for Middle East Strategic Studies, a think-tank close to the office of the Iranian president, said the assassination of Soleimani has made Iranians "rally around the flag".

At Tehran's Valiasr Square, an oversized red banner, covering several floors of a mid-rise building, showed a white silhouette image of Soleimani. "Your blood challenges any adversary," read an inscription in English underneath his image.

At the Tehran Metro, passengers were heard chanting, "No compromise, no surrender."

Soleimani's remains were originally scheduled to arrive in Tehran on Sunday night. But the ceremony in Mashhad was delayed for hours due to the size of the crowd that showed up and paid tribute at the Imam Reza Shrine, the heart of the Shia faith in Iran.

In Tehran on Monday, a sea of men, wearing all-black outfits, and women in black chadors, waited for the arrival of Soleimani's remains, as loudspeakers blared mourning hymns. A traffic jam has formed in the intersections leading to the unfinished grand mosque, named after Khomeini, where the current Supreme Leader Ayatollah Ali Khamenei led the prayers for Soleimani. Khamenei was seen sobbing as he led the prayers alongside President Hassan Rouhani.

Also among those in the crowd was Ismail Haniya, the leader of the Palestinian group Hamas. In his speech, he urged "unity ... to punish those behind the heinous crime", referring to Soleimani's assassination.

On Tuesday, Soleimani's remains will be taken to his hometown of Kerman for the last rites before his burial.