Martin O'Malley, who is seeking the Democratic nomination for president, told the annual meeting of the U.S. Conference of Mayors that the massacre at a black church in South Carolina last week demanded action from Washington.

"I heard some elected officials say this week, 'laws can't change this,'" O'Malley told the group in San Francisco.

"Actually, they can," he said. "How many senseless acts of violence do we have to endure as a people before we stand up to the congressional lobbyists of the NRA?"