Ben Affleck, Matt Damon Pay Respects to Breonna Taylor on Her Birthday

Ben Affleck and Matt Damon are remembering Breonna Taylor on what would have been her birthday ... had she not been fatally shot by cops in Kentucky.

The famous pals got together Friday afternoon and headed into downtown L.A. to honor Breonna, who should be celebrating her 27th today. They left rose bouquets in front of the Hall of Justice ... then walked over to join a protest.

Ben brought his kids Violet, Seraphina and Samuel -- and some of their friends, apparently. The whole gang wore face coverings.

As you know ... Breonna was shot dead by police in Louisville back on March 13th, when cops stormed her home as part of a drug investigation.