Several S.F. Giants players and coaches — including manager Gabe Kapler — knelt for the national anthem Monday … and Donald Trump took aim at them afterward, calling it all a “great disrespect.”

Here’s the deal … before the Giants took on the A’s in an exhibition match in Oakland, Kapler and a few others decided to kneel during the pregame song to protest social injustice.

One of the players involved in the demonstration was Mike Yastrzemski, the grandson of legendary Boston Red Sox Hall of Famer Carl Yastrzemski.

The MLB praised the Giants for the move … defending the team on social media from critics and writing, “The players and coaches are using their platforms to peacefully protest.”

When one Twitter user said the team should not politicize the game … the MLB responded, “Supporting human rights is not political.”

Giants brass also weighed in on the kneeling … celebrating the team for the moves as well, with president Farhan Zaidi saying “we’re proud of our players and staff.”

“We support those who knelt to peacefully protest racial injustice and those who stood to express love of country,” Zaidi said. “We do not see these as mutually exclusive sentiments and believe the freedom to express both is what our country is about.”

But, just hours later, President Trump chimed in on Twitter … and he was furious to see the kneeling protests return to sporting events.

“Looking forward to live sports,” Trump said, “but any time I witness a player kneeling during the National Anthem, a sign of great disrespect for our Country and our Flag, the game is over for me!”

Doesn’t seem the comments are going to deter baseball players … ’cause in San Diego on Monday night, Angels pitcher Keynan Middleton also took a knee and raised his fist during the anthem.

Several other players across the league have also said they’re considering some sort of demonstration when the league officially opens regular season play this Thursday.