In the early stages of the Celtics’ blowout loss in Dallas Friday night, Boston Police arrested a 19-year-old man for pushing an officer repeatedly while “heavily intoxicated” outside of the Game 4 watch party.

Eduardo Caldera traveled from Maine for the festive night on Causeway Street as the Celtics looked to clinch Banner 18, but the Mavericks had other ideas in mind. Dallas romped Boston, 122–84, the third-largest loss in NBA Finals history.

Caldera got separated from his friends who had tickets to the sold-out watch party, and Garden security, around 9:07 p.m., requested assistance from Boston Police to remove the man, seen “heavily intoxicated from alcoholic beverages,” according to a police report obtained by the Herald.

After using a police-issued cell phone to contact his mother, Caldera “pushed an officer despite being warned not to do so,” the report states. “Shortly afterward, he again made physical contact with the officer, prompting the officer to warn him that he’d be under arrest.”

Caldera’s mother, in a phone call, told her son she’d pick him up at a downtown police station, but as officers escorted Caldera out to Causeway Street to take him away, “he pushed an officer once more,” the report adds.

The alleged sequence resulted in Caldera being arrested for assault and battery on a police officer. It marked the only arrest Boston Police made at the watch party, a department spokesperson told the Herald on Saturday.

What could have ended as a celebratory night, if the Celtics pulled off the sweep, was marred by at least one fight among fans in the stands, as Dallas handed Boston its worst loss of the season.

Police officers and Garden security separated at least four fans who got into a fight in the loge, escorting one out of the arena, as the Mavericks increased its lead to 40-plus early in the fourth quarter.

Video of the fight has gone viral, with Barstool Sports and personalities who work for the ultra-popular sports and pop culture media company sharing it on social media.

“Their first loss in over a month and Celtics fans are teeing off on each other at their own watch party,” Barstool Sports posted. Its founder and owner, Dave Portnoy, commented “GUYS GUYS GUYS! . It’s 3-1!!! Poise!!!Poise! POISE!!!

In a separate post later Saturday, Portnoy said: “My concern level with the Celtics is currently 0%. If they lose on Monday it will be 97%.”

Ahead of Game 4, Boston Police geared up for a potential championship celebration outside of the Garden, putting up a series of barricades and warning drivers and pedestrians that streets around the arena would shut down at halftime.

“There will also be no re-entry to any establishments within that area,” a traffic advisory stated.

With the defeat, the Celtics, up 3-1, look to close out the series in Game 5 on Monday. Tickets on StubHub were starting at a whopping $1,347 Saturday afternoon.

Fights broke out in the fourth quarter at the Celtics Game 4 watch party at TD Garden. (Stuart Cahill/Boston Herald)