Videos posted on Twitter showed a security guard pushing and kicking a man away from the pavement outside the Red Lion pub in Westminster.
Pubgoer Stephen Canning posted the videos after witnessing the incident on Friday (31 May) evening while drinking with friends.
Canning recounted the incident to The Morning Advertiser: “There’s a lot of politicians and people like that around the pub, which is right on the Parliamentary estate.
“There’s often beggars going up and down and so it was not uncommon. This guy was coming up and obviously asking for money and the security guard asked him to move on.
“He was moving on and had left and as he was walking away, and had his back to the security guard, the security guard ran up behind him and kicked him in the back, then proceeded to chase him down an alleyway.”
Canning said he followed the pair and stood between them as the guard continued yelling and following the man.
He added: “Everyone was quite shocked.
“I think a lot of people were quite surprised that other members of staff, the pub staff, had not intervened or had got involved or tried to calm anything down.”
A Fuller’s spokesperson said: “When the video was brought to our attention, we made immediate contact with the security company and a thorough investigation is under way.”
A spokesperson for security regulator SIA said it would not comment on individual cases but was looking into the incident.
The security guard pictured was wearing a jacket with the regulator’s branding on it.
SIA said: “We are aware of the incident being reported as a homeless man has been kicked and pushed by a security operative. Violent conduct is unacceptable.
"Protecting the public is our priority and all SIA-licensed security operatives are trained in conflict management.
"This includes defusing conflict and the communication skills necessary to reduce the risk of situations escalating.
"We do not comment on individual cases but are working to identify the security operative in the footage.
"When the licence holder’s identity has been confirmed by a reliable source such at the police or a venue, we will take appropriate action in line with our licensing conditions."
'Out of order'
Thousands shared the series of tweets, with one calling the guard's actions "bang out of order".
The Red Lion looks to soon be consumed into the new Parliamentary estate, plans which its pubco have said could be detrimental to its trade.
The central London borough has one of the highest numbers of homeless people in England, with one in 30 people homeless according to research from charity Shelter published last year.