Iran's late President Ebrahim Raisi is set to be buried in the holy city of Mashhad on Thursday, four days after he was killed in a helicopter crash along with foreign minister Hossein Amirabdollahian and six other people.

Raisi's coffin was flown to Mashhad in northeast Iran after a funeral procession was held for him on Thursday morning in the eastern city of Birjand, where thousands paid their respects as his remains were driven through the streets in a motorcade.

A guard of honour stood to attention as the plane carrying Raisi's coffin arrived in Mashhad, his hometown, where he will be laid to rest at the gold-domed Imam Reza shrine, the holiest Islamic site in Iran and revered by Shi'ite Muslims as the resting place of the 9th century Imam Ali al-Reza.

Raisi, 63, was widely seen as a candidate to succeed 85-year-old Supreme Leader Ali Khamenei, who wields ultimate power in Iran. Mohammad Mokhber, who had been first vice president, is serving as interim president until a June election.

Eight passengers and crew were killed when the helicopter crashed in mountainous terrain near the Azerbaijan border.

A ceremony was held to commemorate Amirabdollahian at the foreign ministry in Tehran, where acting Foreign Minister Ali Bagheri Kani described him as a martyr who had "guaranteed the revolutionary nature of the foreign ministry".

Amirabdollahian will be buried south of Tehran in the Shah Abdolazim shrine, a mausoleum where notable Iranian politicians and artists are buried.

Iran proclaimed five days of mourning for Raisi, who enacted the hardline policies of his mentor Khamenei aimed at entrenching clerical power, cracking down on opponents, and adopting a tough line on foreign policy issues such as nuclear talks with Washington to revive Iran's 2015 nuclear pact.

The presidential election has been scheduled for June 28.