Hundreds of people packed a sweltering Emanuel African Methodist Episcopal Church in Charleston for an emotional memorial service on Sunday just days after a gunman shot dead nine black church members.

"We are reminded this morning about the freshness of death that comes like a thief in the night," the Reverend Norvel Goff told a mostly black congregation that swelled to about 400 people for a service remembering those killed on Wednesday in the latest US mass shooting.

Armed police searched bags at the door of the church, home to the oldest African-American congregation in the southern United States, and officers stood at intervals inside the church along the side of the nave and in the gallery.

Outside the church, a large, mostly white crowd gathered to express solidarity with those inside.

Goff's sermon brought people alternately to tears and laughter as the church reopened to worshippers for the first time since the shooting. They whooped, cheered and raised their hands, and ended the service with embraces after Goff, whose voice rose to a shout at times, encouraged them to "hug three people next to you and tell them, 'It's going to be alright.'"

The suspect, Dylann Roof, was arrested on Thursday and has been charged with nine counts of murder. Authorities say he spent an hour in an evening Bible study group at the church, nicknamed "Mother Emanuel" for its key role in US black history, before opening fire.

Federal investigators were examining a racist manifesto on a website that appeared to have been written by Roof. The site featured white supremacist writings and photos, apparently of Roof.

Goff was standing in for Clementa Pinckney, senior pastor at Emanuel and a Democratic member of the state Senate who was killed in the massacre.

"When evil is in the world, you and I may not be able to control evil-doers. Some of us are still trying to seek answers to what happened last week, Wednesday," Goff said. "I've decided to turn it to over to Jesus."

Among those at the service, which lasted more than two hours, were South Carolina Governor Nikki Haley, US Senator Tim Scott, Charleston Mayor Joseph Riley and Republican presidential candidate Rick Santorum.

"The blood of the Mother Emanuel Nine requires us to work until not only justice in this case but for those who are still living in the margin of life, those who are less fortunate than ourselves, that we stay on the battlefield until there is no more fight to be fought," Goff said.

Hand fans fluttered as those in attendance tried to beat the heat.

"I thought the service was comforting, refreshing and encouraging," said Everald Galbraith, 58, president of the Methodist church in Jamaica, who attended the service. "There was not a sense of great mourning. They recognised what had happened but there was confidence in the salvation of those that died."

The massacre has again trained a spotlight on the divisive issues of race relations and gun crime in the United States and reignited a debate over gun control in a country where the right to own firearms is constitutionally protected.

On Sunday evening about 150 people gathered for a prayer vigil outside the church. At about the same time, a line of people joined hands across the Arthur Ravenel Jr. Bridge, a span that links Charleston with its neighbouring suburb of Mount Pleasant in an event meant to show a united community.

GUN CONTROL

Riley, on the CNN program "State of the Union," called for stricter gun control laws.

"It is insane the number of guns and the ease of getting guns in America," Riley said. "It's not that people should not carry guns and all of that, it's just that there are so many of them and the ease of them and there is no accountability."

President Barack Obama, in an interview recorded on Friday, expressed frustration over the issue. He blamed the powerful National Rifle Association gun-rights lobby group and public apathy for the failure to implement new gun control measures.

The church massacre has also renewed the controversy around the flag of the pro-slavery Confederate Southern states in the American Civil War that ended in 1865. It is a symbol of Southern pride for some and an emblem of hatred for others.

On NBC's "Meet the Press," James Clyburn, a black Democratic U.S. congressman from South Carolina, called on state lawmakers to pass legislation to remove the flag from the state capitol grounds, where it is mandated by law to fly.

The church shootings were the main topic at other Sunday services in Charleston.

At the predominantly white-membership St. Michael's Church, founded in the 17th century, the Reverend Alfred Zadig Jr. said he did not know any of the victims and asked for forgiveness "for failing to be a pastor who reaches out beyond my world".

"You and I are so good at compartmentalizing grief," Zadig told his congregation. "Today I'm asking you to feel the unthinkable pain ... This is not God's will. God did not ordain this event to happen to make a point about racism."