The fight, which resulted in Jonathan Denver's death, started when two groups of people began arguing about the rivalry, San Francisco Police Chief Greg Suhr said Thursday.
Two men were arrested after Denver, the son of a security guard, was stabbed around 11:30 p.m. local time.
One of the men, 21-year-old Michael Montgomery, was arrested on suspicion of homicide. The name of the other man taken into custody was not released.
Denver died at San Francisco General Hospital. He was walking away from the stadium and headed to a bar with his father and brother when a verbal disagreement with other men turned violent.
The two groups exchanged words about the Dodgers and Giants rivalry and a fight broke out. The altercation eventually broke up.
Minutes later, the two groups crossed paths again, and a second fight ensued. It was during the second fight that Denver was stabbed. A second man was treated at the hospital and released.
Denver was a plumber's apprentice in Fort Bragg.
“He was just a very nice young man. He wanted to be a plumber,” his employer, Cas Smith, said. “He was very promising. He was one of our best apprentices. It’s sad.”
Montgomery's father said his son told him it was self-defense, and that he was jumped during the fight. He also said his son told him that Denver hit him over the head with a chair.
“We’re obviously very concerned about the situation,” Giants spokesperson Staci Slaughter said, “and trying to understand exactly what happened.”