A 19-year-old uniformed first-aid man was struck with a rifle butt when he called on paratroopers to stop beating an elderly man, the inquiry heard.
Mr Charles Glenn, who was one of the Knights of Malta volunteers who accompanied the Civil Rights march on Bloody Sunday, said a soldier who jumped out of a Saracen armoured car was repeatedly hitting a grey-haired man on the head with his rifle barrel.
"I did not think that the older man had done anything to deserve such treatment and I thought the para was going to kill him," Mr Glenn said. "I recall shouting at the para: `I order you to stop'.
"As I said this, the para threw the old man to one side and aimed his rifle at me. I thought he was threatening me. Then very quickly another para who I thought for some reason had some type of authority and who came from the same vehicle hit me in the chest with the butt of his rifle and I fell back to the ground and felt quite dazed."
The witness described how shortly afterwards he went to the aid of a young man who was lying on his back in the Rossville Flats car park. This casualty, later identified as the teenager, Jackie Duddy, was unconscious and had lost a lot of blood.
Mr Glenn said he found a bullet entry and exit wound, and concluded the wound was fatal and there was nothing he could do. Soon afterwards, he helped carry the body out of the Bogside in a small group led by Father Edward Daly carrying a white handkerchief.
The witness described how, in spite of his distinctive uniform, he was arrested later by paratroopers, and regimental (military) police took away his first-aid kit and glasses. He was taken by truck, with a number of civilians, to Fort George army base, where they had to run the gauntlet of two lines of soldiers who attacked them with gun butts as they ran to the door of a building.
"It was terribly noisy as we entered the building since the paras outside were screaming at the tops of their voices while inside we could hear dogs barking fiercely. It sounded like hell," said the witness.
He said that, while in the holding pen, he saw an officer whom he recognised from television later as Lieut Col Derek Wilford, OC of the 1st Battalion of the Parachute Regiment. "He proceeded to have a good look at each of us in the holding pen, tilting our heads back by pulling our hair," the witness said. "He appeared to be looking for someone in particular."
Mr Glenn said he thought he also saw Maj Gen Ford (Commander of Land Forces, Northern Ireland) in the holding pen - "He was wearing a peaked cap and battle dress. He was almost amiable and chatty. I think I mentioned that someone needed medical assistance and he said he would see to it."
The witness said a paratrooper then took him to a military policeman and said he had seen him throwing stones, and he was told he was being charged with an offence, "something like riotous behaviour". He was then released.
Mr Charles Clarke QC, for the tribunal, read to the witness a statement from arrest document files of the time. In this statement, a soldier who is photographed alongside Mr Glenn said: "I saw a male person with blue overalls throwing stones at my patrol".
Asked by Mr Clarke if he had been throwing stones at any stage that day, Mr Glenn replied: "Absolutely not". It would be "grotesque" for someone in his position to be throwing stones, wearing a blue overall with prominent badges on it. Mr Glenn will continue giving evidence today.