The 41-year-old victim, identified as Bryan Stow, was hospitalized with serious injuries. According to his family, he is in a medically induced coma, and doctors may have to remove a portion of his skull due to brain swelling.
Police said the incident began as a verbal fight inside the stadium between a group of Giants fans and a group of Dodgers fans. It turned brutal after the game in the parking lot when the Giants fans tried to walk away.
According to authorities, the victim was attacked from behind.
"He fell to the ground and hit his head, and they began to kick him several times," said P.J. Morris of the Los Angeles Police Department.
The victim was punched and kicked repeatedly in the upper torso and in the head.
After the beating, the suspects fled the scene, Morris said. The victim, who is from Northern California, was taken to a local hospital in critical condition.
The attack put a dark cloud over the Dodgers 2-1 win over the World Series champions. Fans from both teams say rivalries should never go this far.
"That's going overboard. They shouldn't have put hands on anybody. They shouldn't have went that far. It's a game," said Diana Cisneros, a Dodgers fan.
Mark Vaughan and his wife came from Northern California to attend the opening day game. They described their experience as the scariest five hours of their lives.
"A lot of people were saying, 'You guys are going to get jumped,' 'You're going to get your butts kicked' in more colorful language that I can't say on television," Vaughan said.
Police said despite Thursday night's attack, Dodgers games are safe for fans to attend.
"We want the fans of the Dodgers and the Giants and the citizens of Los Angeles to know that that environment is safe. We want to make it that way and keep it that way," Morris said.
"It is extremely unfortunate that this incident took place on what was otherwise a great day at Dodger Stadium for tens of thousands of fans," said a Dodgers spokesman in a statement. "We're committed to having the most fan and family friendly environment in baseball and will continue to make that a top priority. We are cooperating fully with authorities during their investigation and we wish this fan a speedy recovery."
Police said they have several witnesses, and they will also be reviewing surveillance video. Investigators are confident they will be able to identify the suspects in the next couple of days.
Anyone with information about the attack is urged to call the LAPD Northeast Station at (213) 485-2563, or call (877) LAPD-24-7.