A teenager has been arrested after allegedly tripping up and spitting at a police officer.
Metropolitan Police officers approached a 17-year-old boy and two other males while patrolling West Hampstead following a spate of mobile phone thefts in the area.
Officers explained to them that they were being detained for a stop and search.
However, the three males rode away on their bicycles.
During a pursuit on foot, one officer collided with the 17-year-old after the teenager deliberately slowed down his bike to cause the collision, police said.
The teenager was arrested and on his way into police custody is said to have tripped up another officer before spitting in his face.
A second 17-year-old boy, who allegedly threatened the police while the other 17-year-old was being arrested, was also arrested for obstruction and public order offences.
Both teens were released on bail while the investigation continues.
Detective Inspector Eliot Porritt, said: “These officers were simply out doing their job, trying to prevent crime in an area known for phone thefts when they were subject to this disgusting and vile behaviour.
"No person should be assaulted whilst at work. We will not tolerate such actions and foul language against officers who are on duty working to tackle violence and protect the public from being victims of crime.
"We will work with our detectives, our partner agencies and the Crown Prosecution Service to ensure those responsible are brought to justice.”