A watch party it was, emphasis on “party.”

As the Knicks were beating the Celtics, 121-113, on Monday night in Game 4 of the Eastern Conference semifinals, joyous chaos reigned outside Madison Square Garden.

Fans watching on three big screens set up on 33rd Street turned the fourth quarter into an extended celebration of their favorite team’s 3-1 series lead.

“I feel so much joy, elation,” Schamgar Cenat of Queens said after entertaining fellow fans by waving a Knicks towel.

He had to be good-naturedly asked to climb down from a planter by security guards.

“I have to be up at 4 o’clock in the morning for work,” he said. “But I don’t care!”

Luis Meneses of the Bronx, who had arrived long before tipoff to get a good viewing spot, said, “I feel like we’ve got it wrapped up in the bag right now. The way [Jalen] Brunson performed, and everyone contributed, it was amazing.

“The atmosphere was amazing. Everybody kept in the game. We stood toe-to-toe with them throughout the game. Let’s go Knicks!”

The party really got going when Mikal Bridges scored with 3:22 left to make it 111-104, inspiring loud choruses of “Go, New York, go! Go, New York, go!”

When OG Anunoby dunked shortly thereafter and Celtics star Jayson Tatum went down with what looked to be a serious leg injury, the delirium reached a new level.

Fans had been booing Tatum every time he appeared on the screen. Many cheered as he was shown being taken to the locker room in a wheelchair.

The crowd seemed generally well-behaved, high-fiving one another and shooting video of each other’s celebrations.

The fun began hours before the game, as fans filtered in early, many dressed for the occasion in Knicks jerseys of various colors and vintages.

The mood was festive, given the lovely weather and the company of fellow fans. Part of the fun was loudly booing every Celtics fan who walked by.

There were screens set up on the east end of 33rd street, and a hoops-themed street fair on the west end.

Before the game, Meneses said he had been to the watch party for every playoff game this season, including one inside the Garden for Game 1 at Boston.

“The energy is electrifying, just like inside the Garden,” he said. “I highly recommend everybody who wants to come to the Garden and watch a game on game days come over . . . It’s very fan-friendly.”

The cost also is friendly: It’s free.

ESPN New York radio’s afternoon show broadcast from 33rd Street. After throwing small sponge basketballs to the crowd during a commercial break, host Don La Greca also praised the vibe.

“To be in the middle of the crowd like this right next to Madison Square Garden, it’s awesome,” he said. “It’s pretty amazing stuff. These fans really deserve to see this team finally win.”

Winning was going to be the most challenging part of the evening. But fans were cautiously optimistic.

“The winner of this game definitely wins the series — that’s a fact,” Craig Sanders of the Bronx said.

It was not a fact, but it was a common opinion in NBA circles. With Tatum now hurt and the Knicks up 3-1, perhaps Sanders will be proven correct.

Pierre Ovalles of Staten Island arrived in a Brunson jersey, Knicks boxing gloves and a crown. He described himself as “50% Dominican, 50% Knicks fan.”

Ovalles said he has been to every watch party this playoff season.

“I won’t lie, it’s exhausting,” he said. “I’m just coming from work and I live in Staten Island. That’s a two-hour commute. But 30-plus years as a Knicks fan, I have nothing but love for the Knicks.”

Four hours later, there was nothing but love for the Knicks on 33rd Street.

Expect the same on Wednesday for Game 5. Cenat, Meneses and Ovalles all said they will be back for more.