TEHRAN — Massive crowds showed up Saturday for the official start of the funeral of Iran’s former Supreme Leader Ayatollah Ali Khamenei in Tehran, Iran’s capital, the beginning of a weeklong mourning period that is meant to project the strength and resilience of the Islamic Republic.

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Draped in the Iranian flag and topped with his black turban, the 86-year-old’s coffin was enclosed within a glass case at the Grand Mosalla, a vast mosque and prayer complex in central Tehran. It was placed alongside the four coffins of his family members, killed alongside him in the opening salvo of the war with the U.S. and Israel four months ago.

Dressed in black, the crowds of mourners started gathering on the streets in the dark and early hours of Saturday morning, after calls from the country’s leaders for them to show up en masse.

Hours later, after the sun rose, their numbers had swelled to tens of thousands. A sea of people was visible inside the Grand Mosalla’s vast courtyard, in front of a colossal banner depicting Khamenei, referred to by many as a martyr. Men wailed in mourning and beat their chests in song, many of them waving or wearing Iranian flags.

The presence of police and security was high in the capital, with roads near the funeral site closed to traffic, forcing many to continue on foot. Loudspeakers broadcast religious songs and music across the capital as many mourners prayed in the streets lined with giant portraits of Khamenei.

Supporters, who travelled in from all over the country, have embraced flag-waving in a way rarely seen before, with many carrying national flags rather than religious banners that traditionally dominate gatherings, reflecting a surge in nationalist sentiment since the start of the war.

In footage, some mourners were heard chanting “revenge, revenge,” and “We will kill, we will ‌kill, ⁠he who killed our Imam.” A photo published by Iranian state broadcaster IRIB appeared to show one banner bearing the English-language phrase “#KillTrump”.

Temperatures also hit 93 degrees as water misted from the rooftops to cool mourners in the heat.

Even larger crowds are expected on Monday, when Khamenei’s casket will be taken on a procession through the streets of Tehran. That’s when, according to organizers, 15 to 20 million people could gather in Tehran alone, making it one of the biggest funerals for any world leader.

Khamenei’s sprawling funeral had been delayed due to the conflict. He was succeeded by his 56-year-old son Mojtaba, who has not been seen since the war broke out and is not expected to attend the funeral events, an Iranian official and a Middle Eastern diplomat told NBC News. The younger Khamenei was injured in the attack that killed his father, three sources said, but the extent of his injuries is not known.

Some initial funeral proceedings were held away from the public on Friday, when the late Ayatollah’s body was visited by senior Iranian officials, senior military leaders, and visiting foreign dignitaries.

They include the powerful Parliament Speaker Mohammad Bagher Ghalibaf, who is also the lead negotiator in talks with the U.S.

Ghalibaf on Thursday urged Iranians to turn out “in the millions” for “the greatest and most lasting farewell in human history,” adding the event would demonstrate that Iran “will not remain silent in the face of oppression and arrogance.”

Foreign dignitaries to visit on Friday included former Russian President Dmitry Medvedev and delegations from China, Turkey, Saudi Arabia, India and Afghanistan, according to the official Islamic Republic News Agency.

As the space opens up to the public, security will be a top concern. Militant groups have targeted funeral ceremonies for prominent Iranian figures in the past. An Islamic State attack on a large gathering for the anniversary of the death of Gen. Qassem Soleimani in 2024 killed at least 84 people in Kerman in central Iran.

Tehran’s mayor, Alireza Zakani, has said that up to 20 million people could show up for the sprawling funeral in the capital, according to the Young Journalists Club, which is affiliated with Iranian state television.

The ceremony will continue with funeral events in Qom, the seat of Iranian religious scholarship, on Tuesday and in Iraq, home to two of the most prominent religious sites for Shiites, on Wednesday. Holding funeral events across the border in Iraq will allow the Islamic Revolutionary Guard Corps, the most powerful military, political and economic force in Iran, to showcase their regional prominence.

Iraqi militia groups allied with Iran carried out more than 600 attacks against U.S. diplomatic facilities in Iraq since the start of the U.S. and Israel-led war, according to the State Department and a senior official.

Khamenei will be buried on Thursday in his hometown of Mashhad, which is also the site of the most prominent Shiite religious shrine in Iran.

Richard Engel reported from Tehran; Babak Dehghanpisheh from New York; and Freddie Clayton from London.