Boris Johnson saves filmmaker Franny Armstrong from attack

Boris Johnson came to the rescue of a high profile climate change activist and filmmaker who was being attacked by a group of young girls brandishing an iron bar, it was revealed today.

Franny Armstrong, the director of The Age of Stupid, described the mayor of London as her "knight in a shining bicycle" after he came to her defence as she was walking home in Camden, north London, last night.

She called out for help to a passing cyclist after being surrounded by a group of hoodie-wearing young girls who pushed her against a car, one holding an iron bar.

The cyclist turned out to be none other than Johnson, who has made tackling youth crime a key mayoral priority.

He stopped and chased the girls down the street, calling them "oiks", according to Armstrong, who praised the mayor's intervention.

Johnson returned and insisted on walking her home.

Armstrong 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 – including the Guardian – and personalities.

"I was texting on my phone so didn't notice the girls until they pushed me against the car, quite hard," she said.

"I noticed that one had an iron bar in her hand. It was very frightening. At that moment a man cycled past and I called out for help.

"He said to the girls: 'What do you think you are doing?' He picked up the iron bar, called after the girls and cycled after them. He returned a few minutes later and walked me home.

"He was my knight on a shining bicycle."

Johnson's office confirmed the story but declined to comment on the mayor's actions.

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