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Cellphone video captured a dispute that escalated into a violent brawl involving about a dozen men outside of a Buffalo Wild Wings in Downey, and police on Monday said they were investigating.

Facebook user Dave Amaro posted the disturbing video shortly after the fight, on Sunday afternoon. He wrote that the incident began when some San Francisco 49ers fans apparently “started problems” inside the Stonewood Center location of the nationwide sports bar chain.

On Monday, Downey police said officers were called to the the Buffalo Wild Wings at 193 Stonewood Street about 1:20 p.m. Sunday for a report of two men fighting.

When they got there, the scene was calm and they initially found little to investigate.

But before police arrived, according to Amaro, 49ers fans wanted to beat up a Pittsburgh Steelers fan, who initially walked away.

Then the fans went outside, where at least a dozen men, including several wearing football jerseys, became involved in a fight, the cellphone video showed. Others tried unsuccessfully to intervene and halt the physical altercation.

At various points, punches were thrown, and at least two men were kicked multiple times as they lay on the ground, according to the video. One of the men appeared to have been kicked in the face.

At one point, someone can be heard yelling, “You’re on camera. That’s assault and battery right there.”

Amaro said the men who were beaten were the 49ers fans who started the dispute.

In a news release, the Downey Police Department stated that arriving officers found one man who said he had been involved in a fight with a group of men who had left, but he didn’t want to pursue charges.

The man later came to the station to file an assault report.

Police soon aware of Amaro’s video, and noted that it showed more than just two men involved in the fight.

“The Downey Police Department takes such matters seriously and will diligently work to identify and prosecute those involved,” the release stated.

Anyone with information was asked to call the department at 562-904- 2308 or the lead investigator, Detective Ron Gee, at 562-904-2331.

While it was not clear what prompted the initial confrontation, the 49ers and Steelers had played each other earlier that day in a contest that ended in a lopsided 43-18 victory for Pittsburgh.

The fight was also the second one involving 49ers fans in less than a week.

Four people were arrested on Friday in connection with the assault of a fan wearing a Minnesota Vikings jersey outside of Levi’s Stadium in Santa Clara, local television station KRON reported. The incident occurred Monday.

Video that went viral after being posted to social media helped lead to the arrests, according to the TV station.

KTLA’s Stephen Acosta and Melissa Pamer contributed to this story.