A binman on a runaway lorry that careered in to a packed street shouted at the driver slumped at the wheel: "You're killing people."
Matthew Telford, 46, said he punched Harry Clarke to try to rouse him as he watched their truck veer out of control in central Glasgow.
Mr Telford told an inquiry into the accident three days before Christmas that he got no response from the driver and was not able to access the vehicle controls himself.
He said: "For a very brief moment I thought he was kidding on. He didn't respond so I said, 'Harry, what are you f****** doing?'
"I suppose a bit of panic set in and I started screaming at him. I started punching him on the back."
The refuse collector said the truck hit a bin on the pavement and his voice cracked as he said he thought the vehicle had struck three women to the side of the bin.
He said: "I started saying 'you're killing people, Harry'."
The six people who died in the tragedy were remembered in a minute's silence at the opening of the inquiry today at Glasgow Sheriff Court.
Sheriff John Beckett gave his condolences to relatives at the hearing and excused them from court when CCTV footage was shown of what one lawyer described as the "graphic and horrific" events of December 22.
The footage showed the bin lorry mount the busy pavement filled with shoppers and skim a wall before it returned to the road and collided with cars.
People who escaped the path of the truck could be seen rushing to help the injured.
Erin McQuade, 18, and her grandparents Jack Sweeney, 68, and Lorraine Sweeney, 69, from Dumbarton, West Dunbartonshire, died from multiple injuries after being hit by the truck.
Stephenie Tait, 29, and Jacqueline Morton, 51, both from Glasgow, and Gillian Ewing, 52, from Edinburgh, were also killed as the lorry travelled out of control along Queen Street and towards George Square before crashing into the side of the Millennium Hotel.
The Crown Office ordered a fatal accident inquiry (FAI) into the crash after prosecutors ruled there was no evidence to warrant criminal proceedings.
When the truck began striking people Mr Telford said he was crouched behind the driver's seat, still wearing his seat belt. He said Mr Clarke's slumped body blocked his access to the controls.
He said: "I was just hoping that we would hit something and stop."
When the truck finally came to a halt Mr Telford said he heard the driver groaning and he "was a shade of grey I'd never seen before".
Neither Mr Telford nor his co-worker Henry Toal, 47, who later gave evidence, are licensed drivers themselves.
Asked by Solicitor General Lesley Thomson, who is leading the inquiry, if he felt there was anything he could have done to stop the tragedy, Mr Telford said: "I don't think I could have done anything different but I suppose I'm always going to be asking myself that question."
Both men said they were not aware of any issues relating to Mr Clarke's health and would have informed a supervisor if the driver had told them of any blackouts or dizzy spells.
Mr Telford said he had received no training in what to do if someone became unwell in the lorry but that procedures have changed since the crash, with crews given daily risk assessment sheets each morning.
Sheriff John Beckett earlier offered his condolences to family members and said: ''I can only imagine the shock, pain and anguish which you have suffered following your sudden and terrible loss.
''I extend my sympathy also to all of those who were injured and traumatised.''
The inquiry, which is expected to last six weeks, continues tomorrow.