Ismail Haniyeh’s funeral has got underway with Iran’s parliament speaker Mohammad Bagher Ghalibaf and Hamas official Khalil al-Hayya addressing crowds of mourners in Tehran.

Supreme leader Ayatollah Ali Khamenei led prayers over Haniyeh’s body at Tehran University in the city centre.

Iranian media said a mourning procession would then head towards Azadi Square to the west.

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Iran’s supreme leader Ayatollah Ali Khamenei will lead the prayers at the funeral of Ismail Haniyeh in Tehran on Thursday, Iranian media has reported, a day after the Hamas leader was killed in an attack in the Iranian capital, and as the region braces itself for a feared escalation in tension.

After the funeral Hamas has said Haniyeh’s body will be transferred to Qatar’s capital, Doha, for prayers and burial. Iran will hold three days of national mourning.

Meanwhile in Lebanon the funeral of Hezbollah commander Fuad Shukur will take place, two days after he was killed in an Israeli strike on an apartment building in Beirut that also killed three women and two children and injured dozens of others.

Hezbollah leader Hassan Nasrallah is expected to speak at the funeral. Shukr was an adviser to Nasrallah, according to Hezbollah sources. Israel had blamed him for a strike that killed 12 children, in the occupied Golan Heights on Saturday, which Hezbollah denied.

Shukr’s body was found under the rubble on Wednesday evening, two Lebanese security sources said, nearly 24 hours after the strike that killed him.

Fears have grown that the assassinations – targeting very high-profile commanders in densely populated capital cities – have killed any chance of an imminent ceasefire in Gaza and that the stakes have been raised for Hezbollah and Iran to reassert themselves.

In other developments:

• Avenging Haniyeh’s assassination was “Tehran’s duty” because it occurred in the Iranian capital and Israel had “prepared a harsh punishment for itself” by killing “a dear guest in our home”, Iran’s supreme leader Ayatollah Ali Khamenei said. The New York Times reported that Khamenei had issued an order for Iran to strike Israel directly, citing three Iranian officials briefed on the order. It was not possible to verify the report. Iranian president Masoud Pezeshkian said the country would “defend its territorial integrity, dignity, honour, and pride, and will make the terrorist occupiers regret their cowardly act”.

• Hamas’s deputy chief in Gaza, Khalil Al-Hayya, said Hamas and Iran do not want a regional war, but that Haniyeh’s assassination was a crime that should be punished. In comments at a press conference in Tehran, he also said whoever replaced Haniyeh would “follow the same vision” regarding negotiations to end the war – and continue the same policy of resistance against Israel.

• Al-Hayya said that Haniyeh was killed by a missile that hit him “directly” while he was staying at a guesthouse in Tehran, where he was attending Pezeshkian’s inauguration. Iranian authorities have said they are carrying out an investigation into the “incident”.

• Senior Hamas official Moussa Abu Marzouk said Haniyeh’s death was a “cowardly act that will not go unpunished”, according to al-Aqsa TV. Another Hamas official Sami Abu Zuhri has told Reuters that the killing was a grave escalation that will not achieve its goals.

• Although Hamas blamed Israel for Haniyeh’s killing and members of the Israeli government celebrated it, the country made no official comment on his death. “We are not commenting on that particular incident,” spokesperson David Mencer told a briefing with journalists on Wednesday.

• Benjamin Netanyahu warned that Israel will “make anyone who is against us pay a very heavy price.” In a televised statement on Wednesday evening, the Israeli prime minister did not mention Haniyeh’s killing, but said his country was “prepared for any scenario” and stood “determined against any threat”.

• Haniyeh’s most likely successor is Khaled Meshaal, his deputy-in-exile who lives in Qatar, analysts and Hamas officials said. Under Meshaal, Hamas emerged as an ever more important player in the Middle East conflict due to his charisma, popularity and regional standing, analysts said.

• The US secretary of state, Antony Blinken, said the US was not involved in or made aware of the assassination of Haniyeh. “This is something we were not aware of or involved in. It’s very hard to speculate,” Blinken said in an interview with Channel News Asia during a visit to Singapore.

• Palestinian president Mahmoud Abbas strongly condemned the killing, Palestinian state news agency Wafa reported. Palestinian national and Islamic factions held a general strike in the occupied West Bank and Palestinians took to the streets in mass demonstrations in response to the assassination.

• Qatar’s foreign ministry called the killing a “heinous crime” and “shameful assassination”. Turkey echoed the condemnation. Egypt said Israeli escalation indicated a lack of political will from Israel for de-escalation. Malaysian prime minister Anwar Ibrahim said he condemned “in the strongest possible terms” the assassination of Haniyeh, calling him a “valiant advocate for his people”.

• The UN security council held an emergency meeting in efforts to keep tensions in the region from boiling over. UN chief António Guterres said the airstrikes in Tehran and Beirut were “a dangerous escalation”.

• Speaking at the meeting, Palestinian representative Feda Abdelhady Nasser condemned Haniyeh’s killing saying “Violence and terror are Israel’s main and only currency”. She added, “There is no red line for Israel. No law it will not breach, no norm it will not trample. No act too depraved or too barbaric.”

• China, Russia, Algeria and others condemned Haniyeh’s assassination, which Iran’s UN ambassador called an act of terrorism. Fu Cong, China’s ambassador to the UN, said failure to achieve a ceasefire in Gaza was responsible for worsening tensions.

• Al Jazeera said two of its journalists were killed in an Israeli attack on the Gaza Strip in which a child was also killed. The Qatar-based broadcaster has named them as Al Jazeera Arabic journalist Ismail al-Ghoul and his cameraman Rami al-Rifee. More than 100 Palestinian journalists have been killed in Gaza by Israel, which has been accused of deliberately targeting journalists. It denies the accusation.

• The UK’s foreign minister, David Lammy, and defence minister, John Healey, travelled to Qatar to help drive efforts to end the conflict in Gaza and call for de-escalation in the wider region, the UK government said on Wednesday.