Police opened fire Thursday at the Marikana mine in Rustenburg -- 60 miles northwest of Johannesburg -- during efforts to disperse armed, striking workers who had gathered on a nearby hill, The New York Times reported.
"The militant group stormed towards the police firing shots and wielding dangerous weapons," Commissioner Riah Phiyega said Friday.
The commissioner said police had been unable to repel the miners with rubber bullets, water cannons and stun grenades,
"This is no time for finger-pointing. It is a time for us to mourn the sad and black moment we experienced as a country," Phiyega said.
"Police did everything they could ... but [the miners] said they were not leaving and are prepared to fight," Police Minister Nathi Mthethwa told Talk Radio 702, in Johannesburg.
The mine is owned by London's Lonmin PLC, the world's third-largest platinum producer.
Rustenburg was a host city of the 2010 FIFA World Cup men's soccer tournament.
It was not immediately clear who fired the first shots, the Johannesburg Mail & Guardian said Friday.
South African President Jacob Zuma said he was "shocked and dismayed by this senseless violence."
"I have instructed law enforcement agencies to do everything possible to bring the situation under control and to bring the perpetrators of violence to book," Zuma said in a statement.
Some South Africans expressed horror at the shootings, comparing them to the apartheid era of racial segregation when white security forces used live ammunition against black township protesters.
Police Capt. Dennis Adriao told reporters Thursday officers did their best to handle a volatile situation and accused the miners of opening fire first.
About 3,000 rock-drillers have been on strike for a week, demanding their wages double as part of a battle between two unions seeking to represent them.
Several hours before the bloodshed began, Lonmin warned the miners in a statement the strike was illegal and any worker who did not return to work Friday would be fired.
Angry miners -- who the Mail & Guardian reported were "chanting war songs" and carrying razor-sharp dead fish known as pangas among their weapons -- attacked police first, police said.
Footage on South Africa's 24-hour eNews Channel showed police approaching the weapons-carrying protesters and firing tear gas into the crowd. Gunfire followed.
The footage later showed bodies lying motionless.
"The South African Police Service was viciously attacked by the group, using a variety of weapons, including firearms," Adriao said. "The police, in order to protect their own lives and in self-defense, were forced to engage the group with force."