Boris Johnson rescued a woman from three "feral kids" who were wielding an iron bar, chasing them away on his bicycle, it emerged tonight.

The mayor of London was cycling through Camden, north London, on Monday night when he answered the cry of Franny Armstrong, a documentary maker and environmental activist who was surrounded by a group of hoodie-clad young girls.

Johnson stopped and chased the girls down the street, calling them "oiks". He then returned to walk Armstrong home. "He was my knight on a shining bicycle," she said today.

Armstrong directed the film The Age of Stupid, and is the founder of the 10:10 campaign, which aims to cut 10% of carbon emissions in 2010 and has attracted support from leading firms and personalities.

"I was texting on my phone so didn't notice the girls until they pushed me against the car, quite hard," Armstrong said. "At first it was quite funny, because they were only about 12. Then I saw that one of them had an iron bar in her hand. It was more than a metre long. It was as big as her.

"Then along came a cyclist. And I thought, 'Good, he's a big bloke,' and shouted, 'Can you help me please?'

"He stopped and turned around and I thought, 'Oh, my God, it's Boris Johnson.'

"He asked the girls what was going on, and at first they didn't move, so I said, 'That's the mayor of London!' and they ran off. They must have thought they were going to get in trouble. One dropped the bar, so Boris picked it up and cycled after them. He returned a few minutes later and walked me home, and we talked about 10:10."

On her Twitter feed she described her attackers as "feral kids".

Armstrong admitted she did not agree with Johnson's politics, and had voted for his rival Ken Livingstone in the mayoral elections. But she added: "If you find yourself down a dark alleyway and in trouble, I think Boris would be of more use than Ken."

A spokeswoman for the mayor confirmed that he had intervened to help Armstrong, but declined to comment further.

Johnson is something of a magnet for action when out and about. Earlier this year, he was nearly hit by a speeding lorry while out on his bicycle scouting for locations for new cycle routes. And in July, he fell in a river in Lewisham, south-east London, while trying to help a clean-up operation.