Police say they are searching for a white male suspect in his 20s. Photo: AP

Police issued images from a surveillance camera of the suspect and a vehicle. Photo: Charleston Police Dept

City police chief Gregory Mullen described the attack at the Emanuel African Methodist Episcopal Church as a "hate crime".

Police later issued surveillance images of the suspect - a white male in his 20s - and a vehicle.

The church's pastor, state Senator Clementa Pinckney, is reported to be among the dead.

A prayer meeting was going on at the time of the shooting at about 21:00 local time on Wednesday (01:00 GMT Thursday) at the church in Calhoun Street.

Police issued an initial description of the suspect as white, about 21, slender build and clean shaven and wearing a grey sweatshirt, blue jeans and Timberland boots.

They later said he was seen driving away from the church in a black four-door saloon car.

Speaking at a news briefing, Mr Mullen said: "There were eight deceased individuals inside of the church. Two individuals were transported to [the hospital]. One of them has died.

"It is unfathomable that somebody in today's society would walk into a church when people are having a prayer meeting and take their lives," Mr Mullen said.

A woman who survived the shooting told her family the gunman said he was letting her live so she could report what happened, the Charleston Post and Courier reported.

She said the gunman had sat in the church before standing and opening fire, according to an official from the National Association for the Advancement of Colored People (NAACP).

Charleston Mayor Joe Riley described the shooting as "the most unspeakable" tragedy.

South Carolina Senator Tim Scott tweeted: "My heart is breaking for Charleston and South Carolina tonight."

The Emanuel AME Church

• Oldest African Methodist Episcopal (AME) church in US south
• Referred to as "Mother Emanuel"
• Roots stem from group of free blacks and slaves in 1791
• Denmark Vesey - one of the founders - was a leader of a failed slave revolt in 1822
• Rebuilt in 1891, replacing church damaged by 1886 earthquake
• Civil rights leader Martin Luther King gave a speech at the church in April 1962

The campaign of US Republican presidential hopeful Jeb Bush cancelled an event planned in Charleston for Thursday due to the shooting.

"Governor Bush's thoughts and prayers are with the individuals and families affected by this tragedy," his team said in a statement.

Helicopters were seen hovering above the area and a police chaplain was at the scene.

At one point police asked residents to move away because of reports of a bomb - but police later gave the all-clear.

A group of worshippers was seen praying near the church.

"We want some real answers now," one of the worshippers was heard saying.

The attack comes two months after unarmed black man Walter Scott was shot and killed by a white police officer in North Charleston.

The shooting prompted angry protests and highlighted racial tension in the city. The officer has since been charged with murder.

Charleston was also due to hold a ceremony on Thursday marking the eighth anniversary of another tragedy - the death of nine firefighters in a blaze at a furniture store in 2007.