The Metropolitan Police has begun an internal investigation into allegations of excessive force against a group of men who were arrested after a party.
The incident happened just after midnight on 18 March at The Lexington pub on Pentonville Road, Islington.
The BBC has seen mobile phone footage showing some officers with batons moving forward towards the crowd.
The Directorate of Professional Standards is investigating six complaints made to the Met.
Party-goers claimed police "overreacted" when they dealt with what was a "private affair" among friends and family.
They said there had been a row among some friends after people had left the party but they had dealt with it themselves.
The footage shows Charles Attah shouting at an officer and pleading with him to listen.
The policeman says "get back" and pushes him away. Within minutes Mr Attah, who runs his own security company, is seen on the floor and arrested.
Mr Attah said: "It went from three officers to nearly 10 officers forcing me down on the floor.
"An officer put his knee to my head and grinded my head in and I got pictures of my head bleeding.
"I had to go to the doctors for bruised ribs I've got to see physiotherapist for my shoulder."
Fabien Soazandry, a 24-year-old documentary maker from Lambeth, said: "The police charged in the direction I was.
"They pushed me up against a tree and then one of the police officers picked me up by both my legs and then they power slammed me to the floor.
'Fearful of their safety'
"I landed face first then I don't know how many police officers there was on my back."
Police said two officers on patrol in the area called for back-up when they came across a fight between two people and intervened.
A Met Police statement said: "When officers became fearful of their safety, due to the scale of the disturbance, they requested assistance from further officers who duly attended."
Police said they arrested six men on offences including affray, assaulting and obstructing a police officer and breach of the peace. One has been released and the five others are waiting to see if they will be charged.
All six have lodged complaints against the police for heavy handedness and excessive force and an internal investigation has been set up.
Retired Met deputy assistant commissioner David Gilbertson, who was once Ch Supt in Tottenham where he worked on improving police and community relations, said: "Nowhere on that footage is there any evidence of anyone being in control of that situation.
"It seems that they've heard what was a call for urgent assistance, they've then ignored common sense and all rules of deployment. "
But Peter Smyth, from the Police Federation of England and Wales, said: "We do police by consent and we need the public's consent to do our job.
"But the public also have to accept policing is a confrontational business and you will always get two sides of an argument and police officers have to do their job."