This is the satisfying moment a thug suffered instant karma after bullying an old man - only for Good Samaritans to fight back.

The cocky young man named Hu was entering the lift at a KTV karaoke club when the old man wearing an orange polo accidentally closed the doors on him in Taichung, Taiwan, on July 1.

Despite the unintentional mistake, Hu proceeded to accost the hapless senior, who tried to appease him by repeatedly bowing his head and apologising.

The bullying continued for about several minutes, with the thug threatening to punch him while his friends simply watched as he harassed the pensioner.

Hu later left the lift with his companions but continued to throw his weight around and pester the old man in the lobby.

A portly man in a black suit stepped in after seeing the commotion and confronted Hu as tempers flared even further.

The argument went on until, in a movie-like sequence, several men came rushing out of the lift and pounced on Hu and his friends.

The bully cowered as he was thoroughly battered by the 'Avengers,' who repeatedly stomped on his head and bashed him with what appeared to be a motorcycle helmet as he lay on the floor.

One man was captured on camera giving a final blow to the ruffian's head before leaving him knocked out cold, all his bluster now snuffed by the one-sided beating.

Taichung police arrived at the scene after the brawl and took in Hu, the old man, and the group of vigilantes for questioning at the station.

Officers said the stout man in the black suit called his 'friends working upstairs' for back-up during his confrontation with Hu. They have also debunked claims the old man was a former gangster who had called upon his 'mob' pals.

Meanwhile, Hu reportedly had mild injuries and scratches. Both parties have declined to press charges against each other.

However, a police spokesman said that Hu 'may be prosecuted for violating the Social Order Maintenance Act'.

They added: 'The parties involved in the altercation did not want to proceed with making charges. They settled the disagreement among themselves but officers are still involved with the case.

'The suspects have been interviewed at the Taichung City police station and police will check CCTV to check if any other offences have been committed.'