A woman in her sixties has been hit by a police car close to Walton Lane police station.
Roads were closed off while officers gave her first aid, before paramedics arrived and she was rushed to hospital.
The incident happened shortly before 10.20pm on Friday 3 May, police confirmed.
The woman, a pedestrian, is receiving treatment in hospital for a serious leg injury.
Witnesses reported seeing an ambulance and police cars at the scene. Motorists have been advised to seek alternative routes.
A statement from Merseyside Police said: "We can confirm a pedestrian has sustained a serious leg injury after being involved in a collision with a police vehicle in Walton.
"The incident happened shortly before 10.20pm this evening (Friday 3 May 2019) close to Walton Lane police station.
"The pedestrian, a woman in her 60s, was given first aid by officers at the scene before the arrival of paramedics. She has been taken to hospital for further treatment for a serious leg injury.
"A road closure is in place on the city-bound carriageway of Walton Lane from its junction with Cherry Lane, and motorist are advised to seek alternative routes.
"An investigation into the circumstances of the incident has been launched. Witnesses will be spoken to and CCTV opportunities explored."
Anyone with information is asked to contact Merseyside Police social media desk via Twitter @MerPolCC or Facebook Merseyside Police CC. You can also call 101 quoting incident reference 1074 of 3 May or contact the independent charity Crimestoppers anonymously, on 0800 555 111 or via their online form at: https://crimestoppers-uk.org/give-information/give-information.
