ISTANBUL, Turkey — Iran saw its deadliest day of protests in six months and the declaration of martial law in at least one city Sunday, as hundreds of thousands of people clashed with security forces on the streets of major cities at the climax of a Shiite religious festival.

Four people were killed in incidents, including a nephew of opposition leader Mir-Hossein Mousavi, and at least 300 arrested, according to police deputy commander Ahmad Reza Radan, being interviewed on IRIB. One of those arrested was the son of Mustafa Moein, a prominent reformist who ran for president in 2005.

Witnesses and opposition news sources reported that government forces had shot live gunfire into crowds of protesters during the holiest feast in Shiite Islam. Another four people were killed in Tabriz, according to a reformist news site.

Witnesses described demonstrators constructing street barricades and fighting against security forces for the control of squares. Unverified reports claimed that Supreme Leader Ali Khamenei had been transported to a military base in a helicopter from his residence in central Tehran.

“There’s fighting going on across the north of the city, and the number of killed is far more than the four whose death has been announced,” said one witness who described the kind of weapons used as "Colts and a larger handgun."

"They have been spraying people with bullets."

The reformist Rahesabz news agency reported that officers refused to shoot into crowds as per their commanders’ instructions and aimed at the sky instead. One photograph showed a riot policeman in his black body armor donning a green cap and joining the ranks of protesters. Elsewhere, protesters stripped demoralized officers of their weapons and set their motorbikes afire.

"This is an inferno that can’t be extinguished with just a splash of water," said Delbar Tavakoli, an exiled journalist living in Turkey who has been watching developments from over the border.

Siavush Randjbar-Daemi, an analyst and Ph.D. candidate in contemporary Iranian history at London University, said: “It’s pure political suicide to kill people on Ashura, of all days.

"They become instant martyrs."

The Internet buzzed with grainy video purporting to show demonstrators exchanging body blows with security forces, torching police motorcycles and vans, and fighting for control of key squares and overpasses in the Iranian capital.

Crowds circulated among a sea of jammed honking cars encompassing most of central Tehran, stretching from Tehran’s Azadi Square to Baharestan. By contrast, the miles of Tehran’s labyrinthine bazaar were shuttered and silent for the anniversary.

Protesters adapted slogans from the 1979 Iranian Revolution, shouting “I will kill, I will kill those who kill my brother,” and “This is the month of blood, the Supreme Leader is toppled.”

Amateur footage and photographs showed crowds kicking an isolated Basiji militiaman or pelting a cornered troop of anti-riot police with stones. Under an unidentified overpass, firefighters kept onlookers back from an overturned police van. Witnesses reported seeing tear gas and black smoke drifting over the capital and the sound of live rounds being fired.

"We’re getting into the final stage of the confrontation, and the ruling clique is waving all pretences of respect to faith, tradition and memory goodbye," said Randjbar-Daemi. "Moharram is a month of truce, so the authorities in Iran are violating everything. It's a regime that is feeling and smelling its own demise, ready to embark onto everything in order to avoid the sinking ship from capsizing."

With the exception of a handful of foreign correspondents, journalists have been expelled from Iran since the disputed presidential elections in June.

In the absence of permission to hold their own rallies, Iran’s opposition activists have piggybacked onto regime-orchestrated demonstrations to get their message out. The death by natural causes of Grand Ayatollah Ali Montazeri last week revitalised the movement and witnessed the spread of protests to peaceful parts of the country.