Front line of the 4x4 war zone (also known as the school run)

It's 8.50am in a prosperous corner of north London and the weekday routine of the school run is under attack. A man dressed as a lollipop attendant is charging at a Range Rover stationary at a zebra crossing, brandishing his placard at the stony-faced woman driver.

It's 8.50am in a prosperous corner of north London and the weekday routine of the school run is under attack. A man dressed as a lollipop attendant is charging at a Range Rover stationary at a zebra crossing, brandishing his placard at the stony-faced woman driver.

As might be expected, she is none too pleased. "Why do you need such a big car to take your child to school," he demands, as she races to wind up the window. Seconds later, the top-of- the-range, four-wheel-drive is surrounded by a gang of demonstrators who attempt to hand her a fake parking ticket admonishing her for her poor choice of vehicle.

"Look at that monster. It's a leather-lined Merc with one child perched in the back. It's probably never gone off-road in its life," says Laura Mackenzie, one of the protesters.

The "monster" is followed by a snaking queue of 4x4s, whose owners are clearly irked at the disruption as they drive to deposit their children at the five nearby schools in Hampstead, including the private girl's school opposite the protest. "You'd drive one too if you had three children," says one well-dressed mother, with another one chipping in: "Mine isn't as bad as the really big 4x4s."

Welcome to the Alliance Against Urban 4x4s protest. Campaigners in teachers' robes hand out "bad school reports" to the agitated mums, targeting parents using four-wheel drives for delivering children to schools. Their action is part of a wider campaign against the "Chelsea monster" - named after the area in London - that has, they say, become the scourge of city life. They accuse 4x4 owners of causing excessive pollution, clogging roads and posing a danger to pedestrians and cyclists in urban areas.

Drivers in Birmingham and Nottingham were also confronted by campaigners.

Suzy Edwards, one of the alliance's founding members, says this was the first of many actions against 4x4 owners in a campaign to make them liable for a higher rate of congestion charge and road tax, as well as a ban on mainstream advertising.

The alliance's activists, made up of the Green Party, Friends of the Earth and Transport 2000, have raised concerns after figures revealed that a record number of off-road vehicles was sold last year. Sales last year grew by 12.8 per cent to a total more than twice the number sold 10 years ago. Fewer than one in 20 of the 4x4s sold are used off-road.

Ms Edwards accuses some owners of using their vehicles like giant, environmentally damaging prams. "How much off-road action have these cars had? They have seen about as much action as a pram. They are an unhealthy danger to the streets," she says. The 4x4 controversy has been raging in the US, where research has shown that someone struck by a sports utility vehicle (SUV) is more than twice as likely to die as someone hit by a saloon car travelling at the same speed.

The New York Times reports that one person a week accidentally backs over and kills one of their own children because of the poor design of some of these vehicles.

In London, the Mayor, Ken Livingstone, stopped short of doubling the £5 congestion charge for off-road vehicle owners, despite describing them as "complete idiots". Activists have campaigned against 4x4s in Paris, and pressure is mounting for a ban in some parts of the US. In 2003, California's state treasurer, Phil Angeledis, sponsored a bill to outlaw SUVs.

In Britain, 4x4s have been deemed "unsuitable for city and town use" by the Royal Society for the Prevention of Accidents (Rospa), which says that vehicles such as hatchbacks and people carriers have been found to outperform bigger four-wheel drives in crash tests.

Kevin Clinton, Rospa's head of road safety, says: "They are not suitable for ... the school run. Hatchbacks and people carriers were found to protect occupants in accidents as well as causing less damage to pedestrians and cyclists." Research by Churchill Insurance also showed that SUVs were 25 per cent more likely to be involved in accidents than an average family car.

But for many in Hampstead, the message fell on deaf ears. A smartly dressed father wound down the window of his silver Mitsubishi Shogun and shouted insults at protesters. "What about buses? They're not great for congestion either," he shouts, adding: "You're the ones blocking the roads, not us. Fuck off, greenies."