The rallies came on a night usually marked by Persian new year (Nowruz) festivities, with the authorities apparently keen to prevent any anti-government dissent at a time when people traditionally take to the streets.

Large crowds rallied from the early evening in the capital Tehran and other cities, many waving the Iranian flag, images on state TV showed.

It had earlier broadcast a message from authorities urging people to join religious groups across the country for a "popular gathering to neutralise the potential plots of elements of the Zionist enemy", referring to Israel.

The call came after Israel said it had killed Iran's security chief Ali Larijani, which has not been confirmed by Tehran, and ahead of expected evening celebrations of Chaharshanbe Suri, an ancient Iranian festival of light and fire marked before Nowruz.

Reza Pahlavi, the eldest son of Iran's last shah ousted by the revolution that brought the Islamic republic to power, on Tuesday called for peaceful Chaharshanbe Suri celebrations and for people to "avoid any tension, confrontation, or even approaching" security forces in the streets.

Iran was rocked by mass protests that peaked in January but were met with a violent crackdown.

Images on state TV showed a large crowd of men being led in prayer in Punak square in Tehran, which in January was a site of massive protests full of people chanting slogans against the Islamic republic.

Around 8:00 pm local time, Tehran's main streets were quiet, with the exception of some cars flying Iranian flags.

But the smell of fire wafted over some streets and a few fireworks burst over the city as people whistled joyfully from apartment windows.

- 'Tradition must not disappear' -

Since the outbreak of the war, authorities have warned against renewed anti-government rallies and imposed heavy security measures.

Judicial authorities had on Sunday warned people not to go into the streets as usual for Chaharshanbe Suri.

"To maintain public peace and safety, please refrain from lighting fires or setting off firecrackers" for the holiday, they said, according to the Tasnim news agency.

The news agency on Tuesday published an image of a bonfire burning the Israeli and American flags.

Judiciary chief Gholam Hossein Mohseni Ejei on Tuesday warned that if anyone disrupted public security over the upcoming Nowruz holidays, "they will face strict legal action and there will be no tolerance or forgiveness for these elements."

State TV reported six people were arrested over accusations of being "ringleaders of the monarchist terrorist network" planning unrest on Tuesday. It did not give further details.

A near-total internet blackout imposed by Iranian authorities since the war started has severely hindered access to information inside the country, though some Iranians manage to send messages out through sporadic connections.

A 35-year-old man living in Bukan in a Kurdish-populated region of northwestern Iran told AFP outside the country that despite pressures not to light the traditional fires, the city was preparing to celebrate.

"We know there's a risk of clashes with government forces, but we're trying not to provoke them too much so they don't shoot at us," he said, adding that more security had been deployed in the city centre.

"Our town is famous for its grand Chaharshanbe Suri celebrations. This tradition must not disappear because of the war or for any other reason," he added.

Iranian officials had also called on citizens to join the funeral ceremonies on Tuesday of 84 navy sailors killed when their frigate was sunk by a US submarine.

Israel and the United States launched strikes on Iran on February 28, killing the Islamic republic's supreme leader, Ali Khamenei, and triggering a conflict that has spread across the Middle East.