DUBAI — Iranian President Ebrahim Raisi and the country's foreign minister were found dead on Monday hours after their helicopter crashed in fog, leaving the Islamic republic without two key figures as extraordinary tensions grip the wider Middle East.

Supreme Leader Ayatollah Ali Khamenei quickly named the first vice-president, Mohammad Mokhber, as caretaker.

"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.

Iran has offered no cause for the crash nor suggested sabotage brought down the helicopter, which fell into mountainous terrain in a sudden, intense fog.

In Iran's capital Teheran, businesses were open and children attended school on Monday. However, there was a noticeable presence of security forces.

Later in the day, hundreds of mourners crowded into downtown Vali-e-Asr Square holding posters of Raisi and waving Palestinian flags. Some men clutched prayer beads and were visibly crying. Women wearing black chadors gathered together holding photos of the dead leader.

"We were shocked that we lost such a character, a character that made Iran proud, and humiliated the enemies," said Mohammad Beheshti, 36.

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 flew at half-staff and a black banner was hoisted at a major Shiite shrine in the city of Qom south of Teheran.

A procession on Tuesday led by a semitruck carrying caskets of the dead slowly moved through the streets of downtown Tabriz, the closest major city to the site of the crash.

A funeral procession will take place in the capital on Wednesday.

The election for a successor is to be held on June 28, IRNA said.

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

Last March, Iran and Saudi Arabia signed a surprise deal that restored diplomatic relations.

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

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

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

Hamas mourned Raisi as an "honorable supporter" of the Palestinian militant group, and hailed his "support for the Palestinian resistance".

Agencies via Xinhua