Green Day frontman and Oakland native Billie Joe Armstrong was among the 27,000-plus at Oakland Coliseum on Tuesday night sending a “Warning” to A’s owner John Fisher. 

A’s fans held a reverse boycott as a protest over a proposed move to Las Vegas as they chanted “Stay in Oakland” and “Sell the team” throughout the night. 

Armstrong, a longtime fan of the Athletics, posed for pictures with fans inside Oakland Coliseum and showed his support on his Instagram. 

One story Armstrong posted showed fans chanting and was captioned, “Oakland forever, Vegas Never. #selltheteam.”

Other stories featured scenes from inside the stadium of fan-made signs.

Gabriel Hernandez posted a photo on Twitter of Armstrong, who was holding up a neon green shirt with the word “sell” written across it, 7,000 of which were handed out before the game.

The A’s had their largest crowd of the season — and biggest on a Tuesday since 2018 — with 27,759 fans attending Tuesday night’s 2-1 win over the Tampa Bay Rays. 

Oakland has won seven straight games and picked up back-to-back wins against the best team in baseball. 

The festivities did take a small turn at the end when fans littered the field with trash after the final out was recorded. 

It’s unclear exactly how Fisher felt about the boycott as he declined comment when asked questions outside MLB’s owners’ meetings in New York on Wednesday, according to Newsday.

Oakland fans have criticized Fisher over potentially moving the franchise to Las Vegas, and the boycott proved more poignant when Nevada’s State Senate passed a bill that would provide public funding for the A’s planned stadium in Vegas on the site of the Tropicana hotel. 

The Nevada Assembly is considering the bill Wednesday and, if passed, would go to the desk of Gov. Joe Lombardo for his signature. 

The stadium would seat 30,000 fans, which would make it the smallest stadium in Major League Baseball.