Thousands of mourners gathered Sunday in the centre of Iran's capital, while some celebrated elsewhere after the death of Iran's supreme leader, Ayatollah Ali Khamenei.

The mourners, dressed mostly in black and some crying, chanted "death to America" and "death to Israel" in Enqelab (Revolution) Square, with many waving Iran's flags and holding photos of Khamenei.

Iranian state television announced a 40-day mourning period and seven public holidays.

"With the martyrdom of the supreme leader, his path and mission neither will be lost nor will be forgotten; on the other hand, they will be pursued with greater vigour and zeal," a presenter said.

Iran's President Masoud Pezeshkian on Sunday branded Khamenei's killing a "declaration of war against Muslims" by Israel and the United States.

"The Islamic Republic of Iran considers it its legitimate duty and right to avenge the perpetrators and masterminds of this historic crime," Pezeshkian said in a statement carried by state TV.

Must avenge blood

Iran's powerful Revolutionary Guards vowed earlier on Sunday to punish Khamenei's "murderers."

In the holy city of Qom, hundreds gathered at the shrine of Hazrat Masume to denounce the attacks.

In Mashhad, mourners observed a symbolic gesture of grief by draping a black flag over the dome of the Imam Reza Shrine, one of Iran's most revered religious sites, and many were seen shedding tears in the surrounding area of the shrine.

At a rally mourning Khamenei's death in Tehran, one man said the news had filled him with hatred "towards Israel and America. We must avenge the blood of the leader."

"We said last night until the morning that God willing, it is a lie. Unfortunately, it was the truth," said a woman mourning Khamenei's death, in a pool ⁠report via WANA.

Atousa Mirzade, a primary school teacher in ⁠the central city of Shiraz, said she could not be happy about ‌the country's leader being killed by a foreign power.

"I also cannot be happy because I don't know what will happen to our country. We ​saw what happened in Iraq – chaos ⁠and bloodshed. I would prefer the Islamic Republic to that situation."

University student Hossein Dadbakhsh, 21, ⁠in Mashhad, said Iran would avenge its leader.

"I am ready to sacrifice my life for Islam and for ⁠my Imam Khamenei. The Zionist regime ​and Trump will pay a heavy price for the martyrdom of my leader," he said by phone, his voice trembling with emotion.

Khamenei's rule faced numerous waves of unrest over the decades, including student-led protests in 1999 and 2002, unrest in 2009, and the more recent "Woman, Life, Freedom" protests of 2022, triggered by the death of a young woman while she was in the custody of ​the morality police.

Some celebrate killing

A 33-year-old woman from Isfahan, meanwhile, said she began crying from a mix of joy and disbelief when ​she heard Khamenei was dead.

Some took to the streets cheering with joy and playing celebratory music after reports of Khamenei's death, according to witnesses and video footage verified by AFP.

There were celebrations in Tehran shortly after 11 p.m. (7:30 p.m. GMT), even before Iranian state television had confirmed U.S. President Donald Trump's statement that Khamenei was dead, according to multiple witnesses and audio recordings.

Loud and sustained whistles, cheers were heard and fireworks could be seen being launched on videos shared on social media that were verified by AFP.

People were not, however, coming out en masse to celebrate, according to social media.