Protests have broken out in Philadelphia after police shot and killed a 27-year-old black man they said had been armed with a knife. The shooting happened on Monday afternoon when two officers responded to a report of a person with a weapon. Police said the officers opened fire when the man, identified as Walter Wallace, "advanced towards" them. An investigation is being opened. Hundreds took to the streets to protest against the shooting. The protests, which began late on Monday and continued into the early hours of Tuesday, turned violent at times, the Philadelphia Inquirer newspaper reported. Images from the protests showed a police vehicle set on fire. There were also reports of looting. Some 30 officers were injured, most of them from being struck by objects such as bricks and rocks, according to preliminary information from police cited by the Associated Press news agency. One officer was in hospital with a broken leg and other injuries after being struck by a pickup truck, police said. Dozens of arrests were made, according to reports.