Soldiers from the Parachute Regiment who carried out the "appalling slaughter" on the streets of Derry on January 30, 1972, also brutally mistreated their prisoners, it was alleged.
The claims were made by a former major in the 1st Battalion of the Coldstream Guards at the central London inquiry into the deaths. The officer, known only as INQ 179, recorded in his diary on the day that he was horrified by the Paras' actions, adding: "Words cannot describe what a dreadful and ghastly regiment that is."
He wrote the next day: "Yesterday is already being called 'Bloody Sunday' and indeed it was.
"There is something quite horrible in seeing young men shot down by totally undisciplined troops, who take a pride and pleasure in this legalised murder.
"I saw the snatch squad of the Parachute Regiment (1st Battalion) bring in civilian prisoners the way these savage, trained terrorists treated those civilians was beyond description."
The soldier, who worked in the communications room at the Fort George camp in the city, recalled seeing the paratroopers return with civilians. He said in his statement that the soldiers, who later claimed they opened fire at the protesters in self-defence, were "screaming and yelling" at them and may even have been hitting or kicking them. Although he did not intervene, he said he was so appalled he told his commanding officer, who in turn told him to record the details in a report.
The officer later wrote in his diary that the paras' behaviour to the civilians was "ghastly and frightful" and that the events had "drastically" changed his mind about the Northern Ireland situation.
He added three days after the deaths: "Nearly all the 13 people murdered were young men in their prime of life under 20 years of age.
"It horrifies me to think of a young body destroyed at such an early age.
"Sunday was the shame of the [British] army in Ireland. We will never live it down."