The 57-year-old, best known for his role in ITV drama DCI Banks, is accused of inflicting grievous bodily harm on Karl Poole on 30 May 2021.
Newcastle Crown Court heard he punched Mr Poole to the ground, causing him to bang his head and fracture his skull.
Mr Tompkinson, who lives in Whitley Bay, North Tyneside, denies the charge.
Prosecutor Michael Bunch told the court the actor had called the police at about 05:30 before approaching two drunk men.
He told officers he had taken a bottle of Jagermeister from them and wanted them "moved on" as they were causing a disturbance outside the house he shared with his partner and her child.
In the call, played in court, he said he had "two incapable drunks" outside his house and that one of them was "just in his underwear".
"I've asked them to move. They can't move," he told the call handler.
The jury heard Mr Poole and his friend, Andrew Hall, had been drinking since around midnight and had gone to the beach to watch the sunrise before walking back to Mr Hall's house, passing Mr Tompkinson's home on Beech Grove on the way.
A neighbour, Caroline Davidson, was woken by the noise of "laughing and carrying on" and looked out of her bedroom window to see the two men lying on a path, where Mr Poole was only wearing underpants with a towel lying next to him.
She looked out again about five minutes later and saw Mr Tompkinson speaking to the men and "formed the view, from his hand gestures" that the actor was telling them to "get on their way", Mr Bunch said.
"It appeared that one of the men said something back and Mrs Davidson saw her neighbour draw back his fist, before apparently thinking better of it and lowering his hand," he said.
The two men were "obviously heavily drunk", Mr Poole "could hardly stay upright" and stopped after walking a short distance before "wobbling from side to side", the court was told.
Mr Tompkinson then "slapped Mr Poole with his right hand before punching him to the head with his left fist", Mr Bunch said.
"Mr Poole stumbled and then fell backwards striking his head on the roadway, where he lay unconscious," he added.
The court heard that on seeing Mr Poole fall, Mrs Davidson told her husband to call for an ambulance.
Jurors heard the actor, who starred as Father Peter Clifford in Ballykissangel, used his mobile phone to record two videos of the men but did not call the ambulance service himself.
Still unconscious, Mr Poole was taken to hospital and found to have a fractured skull and significant traumatic brain injuries.
Mr Tompkinson, who was born in Stockton-on-Tees, told police he had acted in self-defence and only pushed Mr Poole after the men became "aggressive" towards him.
He said he turned to face Mr Hall and, when he looked back, Mr Poole was on the ground, the court heard.
However, the actor's claims that the men had been aggressive were "simply not supported by any of the evidence", Mr Bunch said.
"The truth is that the defendant's story is nothing more than a weak attempt by him to deflect blame onto others for what were his wholly unjustified aggressive actions."
Giving evidence, Mr Poole said he had "absolutely zero" memory of that night and his police statement's description of what happened was "from what I've been told".
Mr Tompkinson's defence, Nicholas Lumley KC, suggested to Mr Poole he "fell as a result of a simple push" and had since exaggerated what happened.
Mr Poole confirmed he had made a compensation claim through the Criminal Injuries Compensation Authority.
The trial, which is expected to last four days, continues.