A video from the G20 protests shows an officer allegedly pointing a 50,000-volt Taser at a group of people lying on the floor of a squat in apparent contravention of police guidelines.

The video, taken by a prone protester, shows officers, believed to be from City of London police, screaming at people to get face down on the floor as one apparently stands with a Taser in a two-handed grip in front of him. Another stands with his baton raised.

The footage is published as The Times discloses that the police officer suspended over a video showing him beating a female protester at the demonstrations was already the subject of a complaint for an alleged similar attack on a woman the day before.

Katie Surridge, 24, claims she was the victim of an unprovoked attack by the police sergeant on April 1, less than 24 hours before he was filmed hitting Nicky Fisher’s legs.

The revelations come as Nick Hardwick, chairman of the Independent Police Complaints Commission (IPCC), said that he had “serious concerns” about frontline supervision of officers at this month’s demonstrations and warned police to remember that they were “servants, not masters”. He is due to appear before the Home Affairs Select Committee.

In the video, taken as police storm the squat, said to be near Liverpool Street station, one officer can apparently be seen walking near to the group of protesters on the floor with his Taser pointing at the ground. When the officer can be seen holding the weapon in a two-handed grip, the protesters are apparently already on the floor and do not appear to pose any serious threat.

It is not known what level of threat the officers faced before the video was recorded that may have led to the Taser being drawn.

The weapons are meant to be used to resolve violent situations with minimum force, allowing officers to stay a safe distance from someone who poses a threat.

For more than a minute several officers can be heard screaming “Face down on the floor — do it now”, while another man, thought to be a protester, says: “Stay calm everybody.”

Tasers have been used in London since 2003, and five months ago the Metropolitan Police Authority said that plans to issue police with thousands of the stun guns could spread fear and damage public confidence in the police.

Ms Surridge, a student at University College London, described being allegedly assaulted by the officer. “One minute I was standing up and the next I was on the floor. I had been pushed to the ground by an officer — I had my back to him and it was totally unprovoked. He didn’t try to justify it at all — initially I was just confused. A group of people shouted at him: ‘You can’t do that to a woman’, but he just walked off.”

A complaint was filed to the Independent Police Complaints Commission on her behalf by Rob Faure-Walker, 27. He later recognised the officer as the one seen on video attacking Ms Fisher.

“I was shocked by the attack — Katie had her back to the policeman and he picked her straight up and threw her on the floor. There seemed to be no reason for it,” Mr Faure-Walker, a geography teacher, told The Times. “I walked straight up to the officer and recorded his badge number. He looked quite agitated, and walked away — but his badge was clearly visible.”

During the attack on Ms Fisher, the officer involved was not apparently displaying his badge number, but sources have confirmed that it matches that recorded by Mr Faure-Walker. A spokesman for the IPCC said that the officer — whose name has not been revealed — was the subject of two separate complaints. The Metropolitan Police declined to comment.

A spokesman for Scotland Yard said that an appropriate policing plan had been put in place. He said: “Tasers are used for incidents or operations involving violence or threats of violence with such severity that officers would need to use force to protect the public, the subject themselves and their colleagues. We will assess the footage once we have had a chance to review it."

Times Recommends

• Margot Fonteyn: In her footsteps
• Forget beautiful people, let's have wonky Brits
• Fundamentalist brew: faith and power