A volley of stones and bricks landed on the embassy gates. Between them and the high brick wall stood an unsmiling cordon of riot police, shields raised and helmets down. "Death to England, Death to America!" shouted the several hundred Iranian student protesters. An occasional loud bang reverberated along the street as another firecracker was loosed.
The police surged forwards, forcing back the demonstrators with batons and occasional squirts of pepper spray. Young men with scarves tied over their faces hung from street signs waving flags and placards. They demanded the closure of the embassy, the expulsion of the ambassador and a trial for the 15 British sailors and marines held captive by Iran.
Mostly drawn from one of Tehran's more hardline universities, Shahid Beheshti, the 200 or so students were having a good time. Away from the press cameras at the centre of the protest, where they looked angry and fierce, they were laughing and nudging each other conspiratorially. "You're English?" asked one with a grin. "Death to England! Ha ha ha."
A slightly older man, from the Basij militia to which most of these students belong, gestured to the diminishing crowd. "Look there are hundreds of people here... or even thousands. It proves Iranians hate Mr Blair."
A bare hundred yards from the officially approved demonstration stood a family of onlookers eating ice creams. A couple of hundred yards further on people seemed barely aware a demonstration was happening. It gave a slightly surreal air to the riot police and the stone throwing. A hubcap sailed over the embassy wall like a Frisbee.
Right in front of the cameras stood the grizzled cheerleader of revolutionary demonstrations, who goes by the nom de guerre Haji Bakhshi. He was surrounded by fierce young men, their eyes lit up with fervour and their fists pumping the air. At the fringe of his raging group militant students were taking photographs with their mobile phones.
Haji Bakhshi is always at demonstrations. Dressed in combat fatigues, and sporting a leonine grey beard, the veteran often leads the chants. After winning a name for blind courage in the Iran-Iraq war, he returned to a country where backsliders had undermined revolutionary values. He now drives around Tehran yelling at improperly dressed girls from a bullet-riddled jeep.
A sudden surge of police pushed the protesters back across the road and gradually traffic started to slide through. After a final few roars of Islamic opprobrium the crowd dispersed, pushed backwards by the riot police. A dozen women students in chadors were left, brandishing forlorn signs demanding the embassy be closed.