Thousands attended the funeral Tuesday of an Iranian Revolutionary Guards colonel who was shot dead in Tehran in what authorities have dubbed an "assassination" by agents of the United States and its allies.

Colonel Sayyad Khodai was killed on Sunday by a gunman on the back of a motorcycle as he sat in his car outside his home.

President Ebrahim Raisi vowed on Monday to avenge the killing -- the most high profile inside Iran since that of top nuclear scientist Mohsen Fakhrizadeh in November 2020.

Iran had accused Israel of masterminding that attack.

The state broadcaster described Khodai as a member of the Quds Force -- the foreign operations arm of the Islamic Revolutionary Guards Corps, Iran's ideological army.

It had previously reported that the colonel was "known" in Syria, where Iran has backed the government during an 11-year civil war and where it acknowledges deploying "military advisers". 

A large crowd gathered Tuesday morning at the Imam Hossein Square in central Tehran to pay their respects to the colonel, with many raising pictures of other "martyrs" killed abroad.

Nearby, a billboard vowing a "harsh vengeance" for Khodai was erected alongside a portrait of General Qasem Soleimani, a revered Quds Force commander who was killed in a US drone strike in Iraq in January 2020.

The ceremony began at around 8:30 am (0400 GMT) with the national anthem and Koran recitations, followed by religious chanters paying tribute to the slain colonel.

Funeral prayers were led by the capital's top imam, Ayatollah Kazem Sedighi. Khodai's coffin was draped in the Iranian flag and posters hailing him as a "martyr".

A military vehicle carried the coffin to Shohada Square, near where he was killed.

Occasional cries of "death to America" and "death to Israel" were heard as the procession wound its way across Tehran.

His body is to be laid at the Behesht-e Zahra cemetery south of Tehran, near the grave of Soleimani's bodyguard, also killed during the 2020 US strike, state television said.

The spokesman for the joint chiefs of staff of the Iranian armed forces, General Abolfazl Shekarchi, said on Monday that an investigation had been opened into the circumstances surrounding Khodai's murder.

The Revolutionary Guards denounced his killing as a "terrorist act", blaming it on "elements of global arrogance" -- in reference to the US and its allies, including Israel.

According to state television, Khodai was born in the northwestern town of Mianeh, in East Azerbaijan province, in 1972.

His funeral was attended by Revolutionary Guards chief Hossein Salami and Quds Force commander Esmail Qaani.