A Sapa reporter at the scene said he counted 18 bodies lying next to the nearby Wonderkop squatter camp. The shooting lasted about three minutes.
Police had tried to intercept a section of the strikers gathered on top of a hill.
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The strikers were wielding pangas and chanting war songs. Police fired teargas and then used a water cannon to disperse the strikers, who retaliated by firing live ammunition at the police.
The area around the hill, which the strikers had turned into their base for the past few days, was cordoned off with barbed wire.
The strikers scattered towards the squatter camp and the nearby veld.
Police spokesperson Captain Dennis Adriao told journalists at the scene police had been "tactical" in their approach and that the situation was tense.
Police officers said talks with leaders of the radical Association of Mineworkers and Construction Union (Amcu) had broken down, leaving no option but to disperse them by force.
"Today is unfortunately D-day," Adriao said.
Ten people, including two policemen, have died in nearly a week of fighting between rival worker factions at the mine, the latest platinum plant to be hit by fallout from an eight-month union turf war in the world's main producer of the precious metal.
On Wednesday, up to 3 000 police officers, including members of an elite, camouflage-wearing riot control unit backed by helicopters and horses, confronted the striking rock-drill operators, but there were no clashes.
Before the police advance, Joseph Mathunjwa, president of Amcu, which has been on a big recruitment push in South Africa's platinum mines, said there would be bloodshed if police moved in.
"We're going nowhere," he shouted through a loudhailer, to cheers from the crowd. "If need be, we're prepared to die here."
The unrest has forced Marikana's London-headquartered owner to halt production at all its South African operations, which account for 12% of global platinum output.
Lonmin said it had lost the equivalent of 15 000 ounces of platinum from the six-day disruption, and was unlikely to meet its full-year production target of 750 000 ounces.
Its London-listed shares were down more than 6% on Thursday, bringing total losses since the violence began to more than 13%.
The Marikana strikers have not made their demands explicit, although much of the bad blood stems from a turf war between Amcu and the National Union of Mineworkers (NUM), the dominant union for the last two decades.
At least three people were killed in a similar round of fighting in January that led to a six-week closure of the world's largest platinum mine, run by Impala Platinum. That helped push the platinum price up 15%.
South Africa is home to 80% of the world's known platinum reserves, but rising power and labour costs and a sharp drop in the price of the precious metal this year have left many mines struggling to keep their heads above water. – Sapa, Reuters
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