Ronald Freeman is accused of attacking Peter Brooksbank outside the notorious Sports and Social Bar.
He allegedly shoved the lawyer to the ground after a disagreement, leaving his head black and swollen.
The pair clashed in December last year outside the bar where they'd both been drinking, Southwark Crown Court heard.
Freeman, 57, an engineering contractor in Parliament, denies assault occasioning actual bodily harm.
Mr Brooksbank told the court Freeman ran at him at high speed, adding: "I tried to reach the door and next thing I know I was shoved from behind and I just went forwards.
"I have a vague recollection of hitting the ground, my head hit the ground, it was just paving stones so was not the most comfortable thing to hit.
"Next thing I know is I was being lifted to my feet by one or two police officers, I do not know how much time had passed."
Mr Brooksbank was taken to hospital and given more than a dozen stitches, jurors heard.
He is a lawyer who works for the joint committee on statutory instruments, a group of MPs and peers who examine legislation.
The court heard he was a regular at the bar and had arrived after 4pm where he had drunk around three pints of Guinness before the incident.
At around 6pm Mr Brooksbank had gone out for a cigarette – which when he first encountered Freeman.
Prosecution lawyer Ollie Welling said the defendant annoyed Mr Brooksbank by repeatedly asking him why he was sitting on the steps.
They then clashed again inside the bar, before Freeman grabbed Mr Brooksbank's parliamentary pass – and the two men then fought in an alleyway, the court was told.
Mr Brooksbank denied having a drinking problem but admitted he would regularly drink eight pints of Guinness when he visited the bar.
He rejected claims that he called Freeman a "peasant" and asked: "Don't you know who I am, I'm a QC don't you know!"
The alleged victim said he reacted badly to Freeman because he "didn't like people touching him".
Charlie Falk, defending, said: "The truth is when he caught up with you, without word, you turned around and punched him in the right eye."
Mr Brooksbank insisted that was untrue.
Sports and Social, which is owned by the House of Lords, has since been closed down and reopened under new management.
The trial continues.
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