Iran held funeral processions Thursday and vowed revenge following the death of Hamas political chief Ismail Haniyeh in Tehran, a strike attributed to Israel.

Supreme Leader Ayatollah Ali Khamenei led prayers for Haniyeh before his burial in Qatar and earlier threatened severe retaliation for his assassination.

In Tehran's city center, thousands of mourners, clutching posters of Haniyeh and waving Palestinian flags, assembled at Tehran University for the ceremony and subsequent procession, according to an Agence France-Presse (AFP) correspondent.

Haniyeh's death was announced a day earlier 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 a.m. (10:30 p.m. GMT) Wednesday.

It came just hours after Israel targeted and killed top Hezbollah commander Fuad Shukr in a retaliatory strike on the Lebanese capital, Beirut, raising fears of a wider regional conflict amid the ongoing Israel's assault on Gaza.

Israel declined to comment on the Tehran strike.

Iran's state TV showed the coffins of Haniyeh and his bodyguards covered in Palestinian flags during the ceremony, which was attended by senior Iranian officials.

Iranian President Masoud Pezeshkian and Islamic Revolutionary Guard Corps chief Gen. Hossein Salami were present.

Haniyeh had been visiting Tehran for Pezeshkian's inauguration ceremony on Tuesday.

Senior Hamas figure Khalil al-Hayya, the movement's foreign relations chief, vowed during the funeral ceremony that "Ismail Haniyeh's slogan, 'We will not recognize Israel,' will remain an immortal slogan" and "we will pursue Israel until it is uprooted from the land of Palestine."

Iran's conservative parliamentary speaker, Mohammad Bagher Ghalibaf, said Iran "will certainly carry out the supreme leader's order to avenge Haniyeh."

"It is our duty to respond at the right time and in the right place," he said in a speech as crowds chanted "Death to Israel, Death to America!"

Our duty

The caskets, adorned with a black-and-white pattern resembling a Palestinian keffiyeh scarf, were carried on a flower-bedecked truck through leafy streets, where cooling water mists sprayed the flag-waving crowds.

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 Islamic Republic has not yet officially disclosed the exact location of the strike.

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

The international community, however, called for de-escalation and a focus on securing a cease-fire in Gaza, which, according to a Hamas official, Israel had previously obstructed.

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

All efforts, he said, should be "leading to a cease-fire" in Gaza and the release of hostages taken during Hamas' Oct. 7 incursion on southern Israel, which marked the beginning of nearly 10 months of war.

The prime minister of key cease-fire broker Qatar said Haniyeh's killing had thrown the entire mediation process into doubt.

"How can mediation succeed when one party assassinates the negotiator on the other side?" Qatari Sheikh Mohammed bin Abdulrahman Al Thani said in a post on the social media site X.

U.S. Secretary of State Antony Blinken on Thursday called on "all parties" in the Middle East to "stop escalatory actions."

Earlier, he said a cease-fire in Gaza was still the "imperative," though White House National Security Council spokesperson John Kirby said the twin killings of Haniyeh and Shukr "don't help" regional tensions.

Tensions inflamed

While Iran has blamed the attack on its arch-foe, Israel has declined to comment on Haniyeh's death.

It did, however, claim responsibility for killing Shukr, whom it blamed for a weekend rocket strike that killed 12 youths in the Israeli-annexed Golan Heights.

The killings come amid regional tensions already inflamed by the war in Gaza, a conflict that has involved Iran-backed groups in Syria, Lebanon, Iraq and Yemen.

One of those groups, Yemen's Houthi rebels, "declared three days of mourning" for Haniyeh, with political leader Mahdi al-Mashat expressing "condolences to the Palestinian people and Hamas" over his killing, according to the group's Saba news agency.

The United Nations Security Council also convened an emergency meeting Wednesday at Iran's request to discuss the strike.

Hamas has been indirectly negotiating a truce and hostage-prisoner exchange deal with Israel, in talks facilitated by Egypt, Qatar and the United States.

Analysts told AFP that Haniyeh was a moderating influence within the group, and though he will be replaced, the dynamics within Hamas could change.

Israeli Prime Minister Benjamin Netanyahu has vowed to destroy Hamas in retaliation for the Oct. 7 attack that ignited the war in Gaza.

That attack resulted in the deaths of 1,197 people.

The group also seized 251 hostages, 111 of whom are still held captive in Gaza.

Concern has grown among Israelis over the fate of those still held in Gaza.

Haniyeh's killing "was a mistake as it threatens the possibility of having a hostage deal," said Anat Noy, a resident of the coastal city of Haifa.

Israel's retaliatory campaign against Hamas has killed at least 39,445 people in Gaza, according to the Hamas-run territory's Health Ministry.