The man suspected of walking into a Charleston, South Carolina church and shooting nine people to death had spent an hour inside with parishioners at a Bible study meeting before opening fire, police say.

“This is a very dangerous individual. He should not be approached by anyone,” Charleston Police Chief Gregory Mullen told reporters.

Mullen also said the dead were six females and three males; the names would not be released until families had been notified.

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A massive manhunt was under way in the South Carolina city for the gunman with police releasing images from security camera footage.

The killings happened at the Emanuel African Methodist Episcopal Church in Charleston, one of the oldest black churches in the US South.

The shooter was described as a white man in his 20s, with a very distinctive bowl-cut hair, wearing blue jeans, a grey sweatshirt and boots, said Charleston police chief Greg Mullen in broadcast remarks.

Police also released a photo of the black sedan he was driving.

Mullen said he had no reason to think the suspect has left the Charleston area, but was distributing information about him and the vehicle around the country.

“He obviously is extremely dangerous,” Mullen said. “We will put all our resources, we will put all of our energy in finding this individual.”

Police are still looking for suspect in Calhoun Street shooting.

— Charleston P.D. (@CharlestonPD) June 18, 2015

The killings have racial overtones and come just two months after unrest broke out in the town of North Charleston, which borders on Charleston, over the killing of a fleeing unarmed black man by a while police officer.

“The only reason someone would walk into a church and shoot people that were praying is hate,” said Joe Riley, the city’s mayor.

Mullen called the killings a hate crime and said the FBI was involved in the investigation because of the “size and scope” of the crime.

South Carolina Governor Nikki Haley called the killings a “senseless tragedy”.

“While we do not yet know all of the details, we do know that we’ll never understand what motivates anyone to enter one of our places of worship and take the life of another,” she said on Facebook.

Scenes from outside the church on Wednesday. Photo: Twitter/@MikeLiggett

Mayor Riley referred to the “unbelievable” slaughter of people praying at the “ritual” of a Wednesday evening prayer meeting.

Wednesday Bible study and prayer meetings are common weekly events in Christian churches across the country.

The year-long US wave of high-profile killings of unarmed blacks by mainly white police officers, starting with the choking of Eric Garner in New York in July 2014, has provoked protests and riots across the country.

Anticipating public outrage over the church massacre, Chief Mullen appealed for calm but indicated he understood the outrage over the crime.

“I understand people are angry (and) upset,” Mullen said. “We’ve had enough of this violence.”

CNN reported that Charleston police had deployed on nearly ever corner of Charleston as the killer still remained at large.

Officials called the killings “unfathomable, despicable, unbelievable”.

Just hours before the shooting, Democratic presidential hopeful Hillary Clinton had stopped in Charleston to campaign. Republican Jeb Bush, who is seeking the centre-right party’s presidential nomination, cancelled his Thursday plans to stop there after the killings.