A lawyer was assaulted after he followed a Houses of Parliament contractor who had yanked his security pass off his neck, a court heard.
Ronald Freeman is accused of shoving Peter Brooksbank to the ground outside a House of Commons bar on 5 December.
Mr Brooksbank said the push left his face "black all over", after hitting his head on paving stones.
Southwark Crown Court heard Mr Freeman 57, of Borehamwood, Hertfordshire, denies actual bodily harm.
The court was told Mr Brooksbank, a lawyer who works with a group of MPs and peers that scrutinises secondary domestic legislation, had gone out of the Sports and Social Bar for a cigarette at about 18:00 GMT.
Prosecutor Ollie Wellings told the jury Mr Freeman "repeatedly asked" why Mr Brooksbank was sitting on the steps, causing the alleged victim, who had drunk three pints of Guinness, to become annoyed.
Later, another "fractious" encounter in the bar resulted in Mr Freeman, a Parliament engineering contractor for 11 years, yanking the pass off Mr Brooksbank, before taking it to another part of the building, said Mr Wellings.
Mr Brooksbank followed the defendant and retrieved his pass before exiting the room, and that he was then allegedly assaulted in an alleyway, the court heard.
In evidence, Mr Brooksbank said he tried to reach a door after Mr Freeman ran at him but "next thing I know I was shoved from behind and I just went forwards".
He added: "I have a vague recollection of my head hit the ground, it was just paving stones so was not the most comfortable thing to hit."
Defending, Charlie Falk put to Mr Brooksbank he called Mr Freeman a "peasant" during in the incident, which he denied.
Mr Brooksbank also rejected the suggestion that he followed Mr Freeman into the restricted engineers' control room, "repeatedly trumpeting" down the corridor "don't you know who I am, I'm a QC".
The trial continues.