Metropolitan Police officers were taking part in an operation to target illegal use of e-scooters in Willesden High Road at around 1pm on Saturday when they asked one rider, who was riding an e-scooter, to stop.
The man failed to do so and crashed into an officer, knocking him to the floor.
The officer treated at the scene before being taken to hospital where it was confirmed he had suffered a broken leg and concussion.
A 22-year-old man was arrested at the scene on suspicion of grievous bodily harm, assault on an emergency worker, obstructing a drugs search, failure to stop, dangerous driving, driving without a licence or insurance, breaching the terms of a prison sentence and being unlawfully at large.
Inspector Martin Robbie said: “Officers put themselves in the face of danger on a daily basis and an incident like this demonstrates this.
“I would like to remind the public that the use of e-scooters, unless subject to a government approved scheme, is prohibited and use upon public highways and spaces will be subject to enforcement and potential seizure of the e-scooter.”
E-scooters can be used on roads, in cycle lanes and on cycle paths, but are banned from being ridden on pavements.
Private e-scooters can only legally be used in the UK on private land, due to an old law dating back to the 1800s, but they are a common sight on roads and pavements.
Parts of London have rolled out a trial of e-scooters: Ealing, Hammersmith and Fulham, The Royal Borough of Kensington and Chelsea, Richmond upon Thames, Tower Hamlets, the City of London, Southwark (north of the borough only) and Lambeth (north of the borough only). Willesden is not one of the areas.
E-scooters are usually limited to speeds of 15.5mph in the UK but e-scooters rented by Londoners are limited to a maximum of 12.5mph in the capital since the trial renting scheme started in June.
As well as having a lower speed limit, the rental scooters will also have lights permanently on, and use geofencing to prevent them being used in areas such as the royal parks. Users will also have to have a safety lesson before their first hire.
Last month, the Metropolitan Police said they had seized 507 privately-owned e-scooters over seven days.