BEIJING — A Beijing man has hanged himself to protest the government’s refusal to account for his son’s death in a military assault on the 1989 Tiananmen Square demonstrations, a group that seeks justice for victims of the assault said Monday.

The group, Tiananmen Mothers, stated on its Web site that the man, Ya Weilin, had disappeared Thursday and was found Friday afternoon hanging in a newly constructed parking garage beneath his home.

The statement quoted family members as saying that Mr. Ya, 73, had written a note earlier that recounted the death of his son, Ya Aiguo, and warned that he would “fight with my death” against the government’s refusal to hear his grievances.

“I can tell that he was in extreme despair when he made the decision,” Ding Zilin, who heads Tiananmen Mothers, said in an interview. “His wife has just gotten out of the hospital for rheumatoid and it must be so hard for him to leave her.”