The party had begun, and no one seemed to want it to end: not the players, not the fans.

For the first time since 1988, the New York Mets are champions of the NL East.

When the Yankees celebrate division titles, they resemble corporate executives closing a deal with handshakes. When the Mets win anything, it’s time to let loose.

“It was like Game 7,” Billy Wagner said after closing out Monday night’s 4-0 clincher over the Florida Marlins. “That atmosphere is what we better get used to, because that’s the way it’s going to be in the playoffs.”

Jose Valentin, one of their unexpected stars, homered twice. Steve Trachsel (15-7), their longest-tenured player, combined with Guillermo Mota, Aaron Heilman and Wagner on a four-hitter that ended nearly two decades of disappointment in their division and days of delay.

“If this is what playoff baseball in New York is like, to me it’s the best,” David Wright said before sticking a cigar in his mouth and high-fiving fans in the first row during the 90-minute postgame celebration.

Fireworks shot off from behind the center-field wall when Cliff Floyd caught Josh Willingham’s fly ball to left for the final out. The Mets rushed to the center of the diamond for a bouncing group hug near shortstop.

“I got that ball in my back pocket. For a small fee, I might give it up,” Floyd said, laughing.

They are New York’s “other team,” often obscured by the crosstown Yankees, whose 26 World Series titles dwarf the Mets’ two.

“If we win the World Series this year, it’s not going to erase the Yankee mystique,” Tom Glavine said. “They’ve done it year after year, and they deserve all the attention they get. We’re just trying to play well and take some of that attention. But sure, we want our piece of the pie.”

The Mets had hoped to clinch last week during a trip to Florida and Pittsburgh. But the 280 or so bottles of Freixenet Cordon Negro Extra Dry sparkling wine that had been flown from city to city were neatly arrayed in four trays outside the clubhouse before the game, as if to tantalize passing players.

By the fifth inning, when the Mets were up 3-0, Pedro Martinez was sitting on the edge of the dugout, wearing goggles — perhaps in anticipation of sprayed sting in the celebration ahead. Possibly remembering the wild celebrations of 1969 and 1986, police deployed four mounted officers on watch behind the right-field fence.

“We accomplished the first step,” Martinez said. “Now we have a big job to do and a great responsibility.”

Led by the power of Carlos Beltran and Carlos Delgado, the pitching of Martinez and Glavine, the spark of Jose Reyes and Wright, and the closing of Wagner, the Mets took over the NL East lead for good with a 2-1 record on April 6 and put together the best record in the major leagues (91-58).

“It was a little bit frustrating because I wanted the guys to experience it right away,” said manager Willie Randolph, a veteran of six World Series titles as a Yankees player and coach.

“It’s that adrenaline that you get. It’s that real euphoric feeling you get knowing that you accomplished something. Having said that, I’ve been through this many times. We have a lot to do, man. We’ve just scratched the surface of how good this team can be.”

Just two years removed from a 71-91 finish that led to the hirings of Randolph and general manager Omar Minaya, the Mets ended the reign of the Atlanta Braves, who had won 14 straight division titles, including 11 in a row since their move to the NL East.

The Mets became the first team this season to clinch a playoff berth and can prepare for their first postseason appearance since 2000, when they won the wild card for the second straight season and lost to the Yankees in the World Series. The Mets are likely to meet St. Louis, Los Angeles or San Diego in the first round, starting in the first week of October.

A giddy crowd of 46,729 chanted, clapped and sang at festive Shea — about 10,000 tickets were sold after Sunday’s loss completed a three-game Pirates’ sweep. Many arrived for batting practice, and fans already were on their feet cheering during the first inning.

Trachsel felt the buzz when he drove into the parking lot.

“Security guards and construction workers were screaming walking in: ‘Get this thing done!”’ he said.

Trachsel, who signed with the Mets in December 2000, allowed three hits in 6 1-3 innings and joined Gary Gentry (1969), Tom Seaver (1973), Dwight Gooden (1986) and Ron Darling (1988) as the only pitchers to win division clinchers for the Mets. Darling was on hand Monday night as a member of the team’s broadcast crew.

Valentin, who took away the second-base job from Kaz Matsui early in the season, hit a two-run homer in the third inning off Brian Moehler (7-9), then earned his second curtain call of the night with a solo shot in the fifth. Floyd added an RBI single in the sixth.

Delgado has the most games (1,703) among active players without a postseason appearance, a distinction that will end in two weeks.

“Now,” he said, “it’s real. We’re going to the playoffs.”