Police bosses said officers worked "decisively and professionally" during an incident at a city train station during which a man was tasered.
A team from British Transport Police was called to Sandhills in Kirkdale on Saturday where a man had "trespassed" onto an out of bounds section of the platform.
Behaving erratically, the bare-chested man was seen flinging his arms in the air, and yelling at officers: "I'm waiting, I'm waiting!"
Witnesses who have gathered to watch the incident tell him: "Chill lad, relax lad."
The footage then shows the Taser, which temporarily disables a person through an electrical current, being discharged.
The officer shouts: "Taser, taser, taser!"
The man is struck and immediately falls to the ground to gasps from the watching crowd.
Today, Chief Inspector David Rams who covers Liverpool for the British Transport Police (BTP), said: "This incident was dealt with decisively and professionally by one of the BTP Mersey team under difficult circumstances being filmed by the crowd.
"Priority as always was to get him the mental health support that he needed.
"We will follow up with any criminal offences."
A British Transport Police spokesperson was unavailable to detail what, if any, criminal offences are being considered.
It is unclear if an arrest was made by officers, although onlookers said the man was led away in handcuffs.
British Transport Police are yet to say why the Taser was discharged, although it is possible it was for his own safety as he was near the tracks.
Merseyrail has declined to comment.
In a tweet yesterday, the operator said: "Due to an earlier trespass incident in the Sandhills area, the 1520 Liverpool Central to Ormskirk service will terminate at Maghull.
"The 1608 Ormskirk to Liverpool service will be started at Maghull."
