Soldiers in Londonderry in 1972
Susan North, who was an assistant to the Italian photographer Fulvio Grimaldi, told the inquiry at the Guildhall in Londonderry that he had just taken his camera away from the window when several shots were fired through it.
She captured the moment on tape, and the recording was played to the Inquiry on Monday morning.
Ms North was giving evidence to the inquiry into the shooting dead of 13 men during a military operation at an anti-internment parade in Derry on Sunday 30 January 1972.
Another man died later from his injuries.
Ms North said she saw injured Peggy Deery being carried to safety and a defenceless and hysterical Michael Bridge shot and wounded in the car park.
Mr Grimaldi then started shouting and swearing at the troops and was "being fired at by them", she said.
Ms North and Mr Grimaldi ran beyond the flats complex through a gap between two blocks only to come across the bodies of Patrick Doherty, Mr McGuigan and Hugh Gilmour.
"Fulvio was busy taking photographs. All I wanted was for the earth to swallow me up. I was deeply ashamed."
She then recalled Alexander Nash - who was shot and injured trying to rescue his fatally wounded son, William - telling how his son was dead at a rubble barricade around the corner on Rossville Street.
'Grief stricken'
She said: "I was so shocked by the things I had seen I did not want to see any more."
The couple then went to a flat to use a telephone and, on the way up, having to climb over the another body - that of Kevin McElhinney - covered by a yellow blanket on the stairwell, two of the teenage victim's young friends sitting beside it.
Ms North told the inquiry, chaired by Lord Saville of Newdigate, that she returned to the spot where Mr McGuigan lay and people were appearing, "shouting, swearing - grief-stricken".
"Even the most hardened person could not fail to have been touched by the monstrous scene," she stated.
Inside the flat, Mr Grimaldi tried to photograph Rossville Street through an open window, only to come under fire as he withdrew.
"This was beyond belief. There were no shots going out that I could hear from the Rossville Flats and there was no reason for anyone to be shooting at the window of the flat where I was in any event."
She added: "The camera that Fulvio had didn't look like anything other than a short lens camera and I cannot imagine that anyone could seriously mistake it for a gun, particularly when the window was only two floors up."
Earlier, Ms North described a man claiming he had a pistol in his pocket as she waited to cross the Rossville Flats car park.
From the witness box, she said: "I didn't like what he was saying - it took my attention away from what was happening."
Counsel to the Inquiry, Christopher Clarke QC, asked: "He said at the time, that he had not used it?"
She replied: "Oh, yes, he definitely said he had not used it."
The Bloody Sunday inquiry was established in 1998, and is expected to run for another two years.