An impassioned President Barack Obama led thousands of mourners in singing "Amazing Grace" on Friday at the funeral of a slain pastor in Charleston and urged Americans to eliminate symbols of oppression and racism, including the Confederate battle flag.

In a speech likely to be considered one of the most memorable of his presidency, Obama paid an emotional tribute to the nine people shot to death at the church and pleaded for Americans to use the tragedy as a way to bridge racial divide.

The shootings last week sparked an intense dialogue over the legacy of slavery and its symbols after photos of the white man charged in the shooting surfaced showing him posing with the Confederate flag on a website that also displayed a racist manifesto.

Politicians and businesses quickly scrambled to distance themselves from the Civil War-era battle flag of the Confederacy amid calls for the flag to be lowered from the grounds of South Carolina's State House.

Obama called the flag "a reminder of systemic oppression and racial subjugation."

"For too long we were blind to the pain that the Confederate flag stirred in too many of our citizens," Obama said in his eulogy for Reverend Clementa Pinckney, 41, of Charleston's Emanuel African Methodist Episcopal church.

At the end of his speech, Obama launched into a rendition of the 18th century hymn, written by a former slave trader after his conversion to Christianity and often associated with African-American struggles. It was a poignant scene for America's first black president who has often been reluctant to play up his racial heritage.

For a moment, he was alone on stage intoning the hymn before purple-clad ministers beside him smiled, stood up and joined him. Then a church organ kicked in and the mostly African-American crowd of about 5,500 people added their voices.

After the hymn, Obama called out the names of the Charleston shooting victims into the microphone. The crowd responded "Yes," to every name. The cadence of his speech was more like that of a sermon than an address.

Obama made frequent reference to God's grace and the failure of the Charleston alleged killer Dylann Roof, 21, to sow bitterness, as witnessed by the forgiveness shown by the victims' families.

"It was a powerful, powerful speech," said David Rivers, 68, a health professor who was in the crowd. "He had a little reverend in him too. Sounded like Reverend Obama," he added.

KILLING WAS PERSONAL

The Charleston killings touched three issues close to Obama's heart: gun control, race and a personal connection to Pinckney, a state senator who he met while campaigning for the White House in the 2008 campaign.

Obama's election raised hopes that the United States was moving beyond racism but the Charleston shooting was another reminder to him and the country that it is still a problem.

"Maybe we now realize the way racial bias can infect us even when we don't realize it, so that we're guarding against not just racial slurs, but we're also guarding against the subtle impulse to call Johnny back for a job interview but not Jamal," he said on Friday.

The Democratic president failed in 2013 in a high-profile effort to have Congress tighten gun laws after the massacre in Newtown, Connecticut, where 20 children and six adults were killed.

"For too long, we've been blind to the unique mayhem that gun violence inflicts upon this nation," he said on Friday. "The vast majority of Americans want to do something about this. ..We see that now," he said.

Obama was joined in Charleston by first lady Michelle Obama and Vice President Joe Biden for the funeral, the third one for a Charleston shooting victim, with the rest scheduled in the coming days.

Even political opponents of Obama were moved by the event.

"I'm a staunch Republican and it brought tears to my eyes a couple of times. I'm not ashamed to say it," said Andrew Smith, Charleston County treasurer.

During his presidency, Obama has spoken at half a dozen memorial services for victims of mass shootings in Texas, Arizona, Colorado and Connecticut.

An accomplished orator, Obama has nevertheless yet to produce an address that would rank alongside former President Ronald Reagan's call for the Soviet Union to tear down the Berlin Wall or some of former President John F. Kennedy's addresses, said presidential historian Thomas Alan Schwartz.

"I don't really think of him as having a great number of speeches like that but he's had these very effective moments at times of national tragedy like at Charleston today," said Schwartz, of Vanderbilt University in Nashville.