More than 27,000 supporters -- the biggest crowd of the season -- filled the stadium to prove to A's owner John Fisher they still support the ball club ... and many wore shirts calling on him to sell the team.

Of course, A's fans have been fed up for years ... with the A's front office seemingly giving up on any hopes of being legit contenders by shipping away young stars and having the lowest payroll in the league -- resulting in poor attendance records at home games.

Now that Fisher has kickstarted the relocation process, the larger-than-normal crowd of A's fans showed their disproval as their beloved team played the Tampa Bay Rays ... chanting "sell the team!" and, at other times, giving the silent treatment altogether.

Armstrong -- an Oakland native and longtime A's backer -- was among the fans in attendance ... and he was all-in on the cause, posting his support on Instagram.

"Oakland forever, Vegas never," the rocker said. "#selltheteam."

BJA was super friendly with his fellow A's fans ... posing for selfies and chatting amongst the crowd throughout the contest.

The A's continued their current winning streak by winning the game, 2-1, but things got ugly shortly after the final out ... with some spectators throwing trash onto the field.