President Obama -- Solos on 'Amazing Grace' (VIDEO)

President Obama really knows how to punctuate an emotional moment -- he belted out "Amazing Grace" ... all by himself during a funeral for one of the Charleston church shooting victims.

The Prez delivered the eulogy for Rev. Clementa Pinckney -- the South Carolina state senator who was gunned down last week ... and as he closed he talked about the call by many to stop flying the Confederate Flag.

He said removing the flag would be an expression of "God's grace" -- smoothly transitioning to the opening lines of "Amazing Grace." Everyone in the arena eventually joined B.O. in song.

Well played, Mr. President.