A group of thugs cheered and filmed on their phones while a police car was smashed in Kirkby.
The marked vehicle was parked on Hollinghurst Road shortly before 4pm on Wednesday before two thugs set on it.
A group of males videoed the attack as the windows of the police car were broken and the incident was recorded.
The footage was then shared online and people in the background could be heard encouraging them.
Just 24 hours later, Merseyside Police confirmed a man had been arrested in connection with the incident.
In a statement, a spokesperson for the force said: "We’ve arrested a 20-year-old man on suspicion of encouraging/assisting an offence after a police car was damaged in Kirkby yesterday, Wednesday, September 30.
"The unattended marked police car was in the Hollinghurst Road area between 3.45pm and 4pm when two offenders caused damage to the front windscreen, side window and back window.
"The car has now had to be taken off the road for repairs.
"It is alleged the incident was videoed on a mobile phone and words of encouragement were shouted at the offenders.
"A 20 year-old man, who is from the Kirkby area, was arrested on suspicion of an offence under s44 of the Serious Crime Act 2007.
"He is currently in custody where he will be questioned by officers."
Knowsley Targeted Inspector Jon Coote said: “Damaging a police vehicle means that it can’t be used which can have huge repercussions on our ability to get to the people who need us most.
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“This type of behaviour will not be tolerated and we will take all available action to find the people responsible for causing damage to police vehicles and anyone who is anyway involved.
“I would ask anyone who has any information about the incident to DM @MerPolCC or @CrimestoppersUK on 0800 555 111 quoting reference 20000592276.”
