Iranians attend the funeral procession of assassinated Hamas chief, Ismail Haniyeh in Tehran, Iran, on Thursday. Reuters

Iran held funeral processions on Thursday for Hamas political chief Ismail Haniyeh after he was killed in a strike in Tehran blamed on Israel.

The Islamic republic's supreme leader Ayatollah Ali Khamenei will lead the prayers for Haniyeh ahead of his burial in Doha, having earlier threatened a "harsh punishment" for his killing.

In the capital's city centre, mourning crowds carrying posters of Haniyeh and Palestinian flags gathered at Tehran University on Thursday morning, according to an AFP correspondent.

Haniyeh's death was announced the day before by Iran's Revolutionary Guards, who said he and his bodyguard were killed in a strike on their accommodation in the Iranian capital at 2:00 am (2230 GMT) on Wednesday.

It came just hours after Israel targeted and killed top Hezbollah commander Fuad Shukr in a retaliatory strike on the Lebanese capital Beirut, sending fears of a wider regional war soaring.

Israel has declined to comment on the Tehran strike.

Ismail Haniyeh attends Iran's new President, Masoud Pezeshkian's swearing-in ceremony in Tehran. File/Reuters

Khamenei, who has the final say in Iran's political affairs, said after Haniyeh's death that it was "our duty to seek revenge for his blood as he was martyred in the territory of the Islamic Republic of Iran".

The Hamas leader was in Tehran for the inauguration ceremony of newly elected President Masoud Pezeshkian on Tuesday.

The Iranian president said Wednesday that "the Zionists (Israel) will soon see the consequences of their cowardly and terrorist act".

Hamas political bureau member Musa Abu Marzuk also vowed retaliation, saying: "The assassination of leader Ismail Haniyeh is a cowardly act and will not go unanswered."

The international community, however, called for de-escalation and a focus on securing a ceasefire in Gaza.

UN Secretary-General Antonio Guterres said the strikes in Tehran and Beirut represented a "dangerous escalation".

All efforts, he said, should be "leading to a ceasefire" in Gaza and the release of hostages taken during Hamas's October 7 attack on southern Israel.

US Secretary of State Antony Blinken also said Wednesday that a ceasefire in Gaza was still the "imperative", with White House National Security Council spokesman John Kirby later adding that the twin killings "don't help" regional tensions.

Agence France-Presse