QUINCY — The young Weymouth man who allegedly murdered his father Monday had attacked his father four months ago during an argument, leaving him with broken ribs, authorities said.

Michael E. Beaudry, 20, pleaded not guilty today in Quincy District Court to a charge of murder for allegedly hitting his father, Ronald, in the head with a three-foot-long piece of PVC pipe, a blow that knocked the elder Beaudry into the street, causing fatal head injuries.

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“Oh, my God,’’ Michael Beaudry allegedly told State and Weymouth police after learning his 58-year-old father had died. “I can’t believe I killed him.’’

The fatal event began Monday afternoon when Michael Beaudry fired off a BB gun in the basement of the family’s home on Cross Street in Weymouth, according to Norfolk County prosecutors and defense attorney Jack Atwood.

The elder Beaudry followed his son, intent on making sure his son did not use the BB gun in the neighborhood as he had in the past, drawing complaints from neighbors, according to both the prosecution and defense.

The younger Beaudry was holding a three-foot long piece of PVC pipe and a small propane gas tank in his hand when he suddenly turned and slammed his father in the head, Norfolk Assistant District Attorney Lisa Beatty.

His father fell to the ground on Cross Street, forcefully hitting his head on the street below, officials said. Ronald Beaudry was rushed to South Shore Medical Center, where he was pronounced dead around 5 p.m.

The victim’s wife, Shirley, was at home at the time. She told police she saw her son run into the house, run into his bedroom, where he grabbed a shirt, then rushed back outside to his stricken father.

Shirley asked her son what happened, according to police.

“I hit him with the pipe,’’ Michael Beaudry told his mother.

When emergency medical providers arrived, Michael Beaudry allegedly pleaded with them.

“You have to save my dad!’’ he allegedly told them.

Michael Beaudry also turned to his father and both apologized to him and also berated him one more time, officials said.

“Dad! Why did you make me do it?’’ Michael Beaudry told his father as he lay on the street. “You should not have come at me. I’m sorry, Dad.’’

Shirley Beaudry was too overwhelmed to make it to court today, but a relative was in court. He declined comment. Also in the courtroom was a small group of men in their 20s who appeared to be Michael Beaudry’s friends. They also declined comment.

The 20-year-old Beaudry also allegedly told police that he and his 58-year-old father agreed on very little and in fact had been “butting heads all the time.’’

According to Atwood, the elder Beaudry was upset that his son was unemployed, used the BB gun in the house and around the neighborhood, and collected World War II memorabilia, especially uniforms.

The younger Beaudry had recently launched an effort to enlist with the US Army and had only a few more procedural steps to complete before he would have left for the service, according to Atwood.

The defense attorney also said that he believed his client should face a charge of manslaughter, which carries a maximum of 20 years in prison, instead of murder which can carry a life sentence.

District Court Judge Mark Coven ordered Beaudry held without bail.