Parents using their 4x4s for the school run were targeted by activists concerned about the environment.
Drivers were given mock school reports graded E for effort
Drivers at a junction in Belsize Park, north London, were stopped in their tracks by members of the Alliance Against Urban 4x4s.
The protesters accused the drivers of being irresponsible but a motoring spokesman said the safety performance was better than many other vehicles.
Drivers of 4x4s in Nottingham, Leeds and Birmingham were also targeted.
Dressed as lollipop ladies and teachers, activists accused parents of "showing off" driving such vehicles and gave them a mock school report graded E for effort.
Siān Berry, a spokesperson for the Alliance, said parents who used such vehicles were being "extremely irresponsible".
"These vehicles are totally unsuitable for cities, jamming up roads, threatening children's safety and increasing pollution.
"It's a real shame that so many adults feel the need to show off to one another like this. Their cars have seen as much off-road action as the average pram."
The Alliance is calling for increased congestion charges in cities and higher road tax for 4x4s which it says creates high levels of local pollution.
Ms Berry added: "The effects of climate change are going to be felt most keenly by the kids who are travelling in the back of these gas-guzzling 4x4s."
But Paul Everett, from the Society of Motor Manufacturers and Traders, said this style of vehicle had become very popular because they were "useful and flexible."
"The environmental performance of your average 4x4 is no worse than a large saloon or people carrier," he told BBC News.
"In many cases the safety performance of these vehicles is better than many other vehicles on the road.
"People do find the raised driving style to be safer, they see more of the road, feel more in control and in many cases drive more responsibly."