A member of the republican movement saw a man with a rifle and heard a shot before he heard shots fired by the Army on Bloody Sunday, the Saville Inquiry has heard.
The tribunal is entering the final stages of hearing evidence
The witness, referred to only as RM 1, said on Tuesday that he remembered feeling "very angry" because of the risk to the crowd.
The tribunal is examining the events of 30 January 1972 when 13 civilians were shot dead by British army soldiers during a civil rights march in Londonderry. A 14th person died later.
He said he was watching a number of young men stoning a small barricade in the area of Little James Street when he heard a shot which seemed to have come from a building behind him.
The republican said as he ran up the stairs in the building, he was thinking: "Who is mad enough to fire a shot with all this crowd about."
He said he grabbed a gun from one of two men, threw the weapon down the stairs and pushed the man after it.
The witness had said in his written submission: "I felt strongly that it should not have happened. I confirm that this was the first shot I heard and I heard no Army fire at the time."
The inquiry has already heard from a former Official IRA member who said he had fired a shot after he heard two people had already been injured by the Army.
Other witnesses have told the inquiry of a confrontation between this man and Tuesday's witness, RM 1.
Soldiers shot 13 people dead in Derry on Bloody Sunday
The hearing of evidence in the Bloody Sunday Inquiry is expected to finish this week.
The last witness is due to testify at the Guildhall in Derry on Thursday.
More than 900 witnesses have testified since the tribunal began hearing the evidence nearly four years ago.
But it will still be some time before the inquiry actually draws to a close.
Counsel to the inquiry, Christopher Clarke QC, will not deliver his closing statement until the autumn and it will be at least this time next year before Lord Saville publishes his final report.
Lord Saville of Newdigate and the Commonwealth judges accompanying him on the Bloody Sunday inquiry began their work nearly four years ago.
The Bloody Sunday Inquiry was established in 1998 by Prime Minister Tony Blair after a campaign by families of those killed and injured.
They felt that the Widgery Inquiry, held shortly after the shootings, did not find out the truth about what happened on Bloody Sunday.