Mets fans greeted Mayor Bill de Blasio with a hearty Bronx cheers on Monday, before he left early on the Amazins’ bitter Opening Day loss.

Donning a pinstripe home white jersey and blue Mets cap over dress pants, de Blasio was loudly booed twice by Flushing faithful — once as he was announced, and then again as he left the Citi Field diamond.

The wave of boos didn’t get to de Blasio, who kept his cool, kicked and delivered from just in front of the pitcher’s mound.

De Blasio — wearing a No. 6 Mets jersey with his name on the back – tossed a strike to Mets catcher Travis d’Arnaud, and flashed a relieved grin that he delivered a corner-painting, Opening Day pitch.

He played off the boos, suggesting that his supporters might have been too cold to cheer.

“As for the response of the crowd, I think everyone is going to be too cold to respond,” de Blasio said, before the Mets’ 9-7 loss in 10 innings to the Washington Nationals.

“But whatever it is, I’m a sports fan. Sports fans have a right to express themselves any way they want.”

De Blasio wore No. 6 in honor of late-1960s Boston Red Sox great Rico Petrocelli, the mayor’s aides said.

The infielder, who slugged 210 home runs in his 13-season career, grew up in Brooklyn. So the Boston-loving mayor managed to pay homage to his beloved Red Sox, to his adopted home borough and to Italian-Americans by donning that shirt.

But that still didn’t get de Blasio off the hook for his war on charter schools.

“I’m not a fan of his — especially on his [positions on] charter schools and pre-K,” said booing fan Jodi Freed, 42, of Bayside.

“Is school a day care or education system? He needs to show more concern by getting rid of Common Core and educating, starting on basics.”

The mayor admitted he practiced for Monday’s big toss with his son, Dante, at the Park Slope Armory.

Glenn Smith, a computer programmer from Kinnelon, NJ, was blown away by the overwhelming boos for the recently elected mayor.

“He’s only been in office for three months. They were pretty hard on him,” Smith said.

“Maybe politicians don’t want to make public appearances because they don’t want to hear the boos. He threw the ball nice, though.”

De Blasio took shelter in cushy Caesars Box seats, Section 311 to be exact, where he noshed on an Italian sausage. And unlike his pizza debacle, de Blasio used his hands, no knife and fork.

He vanished from public view in the fifth inning to hang out with Mets owners in a private suite.

De Blasio left during the seventh-inning stretch, moments after the Nats had scored twice to tie the score, 4-4.

New York would take the lead in the eighth inning before coughing it up in the ninth and losing it in the 10th.

“It shows he has no heart. It’s upsetting [the mayor leaving so early],” said 33-year-old Whitestone resident Matthew Lipsky, a FedEx operations manager who was among the few fans to tough it out through 10 innings.

“He’s supposed to be a leader. What does that show?”

Despite the torrent of boos for de Blasio, bundled-up Mets fans were generally in a good mood, shivering and smiling about baseball’s annual return to Flushing.

The mercury hovered in the high 30s just before first pitch. But it reached a sunny 53 degrees by game’s end.

Retired school teacher Harvey Wiener said he hopes to be shivering at Mets games later this year — in the playoffs.

“This is just like October baseball, so it’s great,” said the absurdly optimistic Weiner, 66.

Even the most jaded Mets fans were excited for the first of 162 games this season. The Queens club hopes to improve from its 74-88 mark of 2013.

“This is greatest day of the year, this should be a national holiday!” said 37-year-old Mike Amendola, a Rockland County PE teacher who had permission from school bosses to play hooky at Citi Field.

The Mets honored beloved late broadcaster Ralph Kiner before the game, unveiling an image of the Amazins uniform patch in left field. The team will wear a patch all season with Kiner’s name over a broadcast mic.

Monday marked the Mets’ first Opening Day without Kiner, who passed away on Feb. 6 at the age of 91.

Bob Morris, 54, and his 19-year-old son, Mike, from Rockaway Township, NJ, held up sticks with Kiner’s face on it.

“We think it’s [the Mets tribute] great. We grew up with him,” the elder Morris said.

“I was just a kid. He is in the history of the Mets. It’s sad because he’s a New York Mets icon and he’s gone now. But I have mixed feelings because there are a lot of good memories, too. He was a guy everyone liked.”

Michael Kiner — son of the slugger-turned-broadcaster — said his old man would have been blown away by the pre-game tribute.

“He would be very humbled by all of this,” the son told SNY. “This would have meant an awful lot of him.”

Even though Ralph Kiner rose to fame as a player in Pittsburgh and lived in Palm Springs, Michael Kiner said his dad’s true love was for the Amazins.

“Even though he was great player for the Pirates, his heart was in New York,” the younger Kiner said. “The Mets fans are the greatest fans in the world and he was a Met.”