CHARLESTON (South Carolina) (REUTERS) - President Barack Obama lead thousands of mourners in singing Amazing Grace on Friday at the funeral of a Charleston church's slain pastor after urging Americans to eliminate symbols of oppression and racism, including the Confederate battle flag, to mend the fabric of the nation.

Obama called the flag "a reminder of systemic oppression and racial subjugation," and pleaded for Americans to be on guard against racial bias.

The shootings last week sparked an intense dialogue over the legacy of slavery and its symbols, centering on the Civil War-era battle flag of the Confederacy and whether it should be lowered from the grounds of South Carolina's State House.

"By taking down that flag, we express God's grace," Obama said in the eulogy for Reverend Clementa Pinckney of the Emanuel African Methodist Episcopal church in Charleston.

"For too long we were blind to the pain that the Confederate flag stirred in too many of our citizens," Obama said.

He led the crowd of around 5,500 people in singing Amazing Grace, a hymn often associated with African-American struggles.

Obama also said that Americans have been blind to the"unique mayhem" caused by gun violence.