Iranians on Monday mourned the death of president Ebrahim Raisi whose helicopter crashed into a fog-shrouded mountain, setting off a period of political uncertainty in the Islamic republic.

Raisi, 63, his foreign minister and seven others died when the aircraft went down on Sunday in a remote area of northwestern Iran, where the wreckage was only found on Monday morning.

The ultraconservative Raisi had been in office since 2021, a turbulent time during which Iran was rocked by mass protests, an economic crisis deepened by US sanctions, and armed exchanges with arch-enemy Israel.

Supreme leader Ayatollah Ali Khamenei, who wields ultimate power in Iran, declared five days of mourning and said vice president Mohammad Mokhber, 68, would assume interim presidential duties.

State media reported late on Monday that an election to replace Raisi would be held on 28 June.

"The Iranian nation has lost a sincere and valuable servant," said 85-year-old Khamenei, whom Raisi had been expected by many observers to one day succeed.

Thousands of mourners massed in central Tehran's Valiasr Square to pay their respects to Raisi and to Foreign Minister Hossein Amir-Abdollahian.

Funeral rites were set to start Tuesday in Tabriz, East Azerbaijan province, for them and the other victims -- three crew, two bodyguards, an imam and a provincial governor -- before Raisi's body was to be taken to Tehran.

Khamenei is expected to lead a prayer at a farewell ceremony for Raisi on Tuesday evening ahead of a funeral procession in the capital the following morning.

Iran's military chief of staff Mohammad Bagheri ordered "a high-ranking committee to launch an investigation into the cause of the president's helicopter crash".

Flags at half-mast

State TV broke the news early on Monday that "the servant of the Iranian nation, Ayatollah Ebrahim Raisi, has achieved the highest level of martyrdom", showing pictures of him while a voice recited from the Koran.

Flags soon flew at half-mast and a black banner was hoisted at a major Shiite shrine in the city of Qom south of Tehran.

Global allies Russia and China and regional powers voiced their condolences, as did NATO, while the UN Security Council observed a minute of silence.

Condolences also came from Palestinian group Hamas, Lebanon's Hezbollah and from Syria, all members of the so-called Axis of Resistance against Israel and its allies, amid high tensions over the Gaza war.

The United States, which has for decades had a fraught relationship with Iran, offered "official condolences" for Raisi's death. 

However, National Security Council spokesman John Kirby said Raisi "was a man who had a lot of blood on his hands", adding he was responsible for "atrocious" rights abuses in Iran and had backed the country's proxies abroad.

Washington had placed Raisi, a former head of the judiciary, on its sanctions blacklist for complicity in "serious human rights violations" -- charges Tehran has rejected.

Analysts did not expect major foreign policy changes, pointing out that ultimate power in Iran is held by the supreme leader and the Islamic Revolutionary Guard Corps, who work with a regional network of armed groups.

"It will be (the) status quo," Jason Brodsky, an expert at the Middle East Institute, told the BBC of Iran's relations with these groups.

"The IRGC reports to the supreme leader and liaises with Hezbollah, the Huthis, Hamas and the militias across the region. The modus operandi and the grand strategy of the Islamic republic will remain the same."

US Defense Secretary Lloyd Austin said he did not "see any broader regional security impact" from Raisi's death.

Lost in fog and rain

Iranian authorities first raised the alarm on Sunday afternoon when they lost contact with Raisi's helicopter as it returned from a border meeting with Azerbaijan's President Ilham Aliyev to inaugurate a dam.

Only two of the convoy's three helicopters landed in Tabriz, setting off a massive search and rescue effort, with multiple foreign governments soon offering help.

Washington said that Tehran had asked it for assistance without providing any details. The State Department said the US had been unable to provide any "largely for logistical reasons".

Interior Minister Ahmad Vahidi at first spoke of a "hard landing" and urged citizens to ignore hostile foreign media channels and get their information "only from state television".

Guards, army and police personnel joined the search as Red Crescent teams trudged up a steep hillside in the rain while rows of emergency services vehicles waited nearby.

As the sun rose Monday, rescue crews said they had located the destroyed Bell 212 helicopter, with no survivors.

State TV reported that the aircraft had "hit a mountain and disintegrated" on impact, and the Red Crescent soon confirmed that "the search operations have come to an end".

Protests and economic woes

Raisi had in 2021 succeeded the moderate Hassan Rouhani, at a time when the economy was battered by renewed US sanctions over Iran's contested nuclear program.

Iran saw a wave of protests in 2022 triggered by the death in custody of Iranian-Kurdish woman Mahsa Amini after her arrest for allegedly violating strict dress rules for women.

In March 2023, regional rivals Iran and Saudi Arabia signed a surprise deal that restored diplomatic relations.

The Gaza war sent tensions soaring and a series of tit-for-tat escalations led to Tehran launching hundreds of missiles and rockets directly at Israel in April.

In a speech hours before his death, Raisi emphasised Iran's support for the Palestinians, a centrepiece of its foreign policy since the 1979 Islamic revolution.

"We believe that Palestine is the first issue of the Muslim world," said Raisi.