At a news conference, St. Louis police Chief Sam Dotson said the suspect held a knife overhead in "a threatening manner" as he approached at least two officers.
It was not clear how many shots were fired, and Dotson did not disclose the race of the man who was shot or of the officers. Both officers opened fire, and no one else was injured, Dotson said.
The incident unfolded in the shadow of Ferguson, where police and civilians have clashed since Aug. 9, when a white police officer shot and killed Michael Brown, an unarmed young black man who witnesses have said had his hands up when he was gunned down.
As Dotson spoke shouts could be heard off-camera, and reporters repeatedly asked the chief if he feared similar unrest in St. Louis. City Alderman Antonio French, who has been heavily involved in the protests in Ferguson, said on Twitter that a crowd was already forming at the scene of Tuesday's shooting.
At one point Dotson was asked if he thought Missouri Gov.
"I think this is an incident that happened in the city of St. Louis," Dotson said.
At least one witness to the incident told police that the suspect may have been intentionally trying to provoke the police into shooting him. Dotson said the man was acting erratically before the shooting occurred.
"Shoot me, kill me now," the man said, according to Dotson.
Both officers have been placed on administrative leave and Dotson said he would try to make information available as quickly as possible. Based on the preliminary facts, though, Dotson seemed to suggest the officer's actions were justified.
"If a person is charging you with a knife and they're within three or four feet, that's a lethal range for a knife," he said.
The man's identity was not released, but Dotson described him as a 23-year-old male.
The shooting came as tensions in Ferguson were entering their 10th day. After days of clashes between police and protesters that made clouds of tear gas and images of riot police commonplace in the
But a series of weekend protests turned violent again, leaving several people injured by gunfire and dozens of protesters in police custody. None of the gunfire came from police, said Highway Patrol Capt. Ron Johnson.
Johnson and St. Louis County Police have both said officers have been the targets of gunfire and Molotov cocktails in recent days, and officers fear a group of outside "troublemakers" have hijacked what had been peaceful protests.