Heavily armed officers backed by armoured vehicles were laying out barbed wire barricades when they were outflanked by some of the estimated 3000 miners massed on a rocky outcrop near the mine, 100km northwest of Johannesburg.
Police opened fire with automatic weapons on a group of men who burst out from behind a vehicle. The volley of bullets threw up clouds of dust, which cleared to reveal at least seven bodies lying on the ground, Reuters television footage showed.
It was not clear whether the police were fired upon. They appeared to be on edge and with rifles pointing forwards immediately before the incident. Reuters photographs showed spears and clubs lying near the bodies.
The SAPA domestic news agency said one of its reporters had counted 18 bodies near a squatter camp close to the mine, Lonmin's flagship platinum plant, which was forced to shut down on Tuesday because of the union unrest.
There has been no confirmation the death toll.
World platinum prices leapt as much as $30 an ounce - more than 2 percent - to a six-day high as the extent of the violence became apparent in the country with 80 percent of known reserves.
Leaders of the radical Association of Mineworkers and Construction Union (AMCU), which was representing most of the strikers, accused police of a massacre.
Some commentators likened the scenes to the pre-1994 days of white-minority apartheid rule, infamous for its pictures and footage of ranks of police opening fire on crowds of black protesters.
"I cannot think of a confrontation between protesters and police since 1994 that has taken place along these lines," said Nic Borain, an independent political analyst.
Before the start of the operation by hundreds of police, officials said several days of talks with AMCU leaders had broken down, leaving no option but to disperse the striking drill operators by force.
"Today is unfortunately D-day," police spokesman Dennis Adriao said.
Prior to Thursday, 10 people - including two policemen - had died in nearly a week of fighting between rival worker factions at the mine, the latest platinum plant to be hit by an eight-month union turf war in the world's main producer of the precious metal.
The Marikana strikers have not made their demands explicit, although much of the bad blood at the mine stems from AMCU's challenge to the two-decade dominance of the National Union of Mineworkers, which is closely linked to the ruling African National Congress.
Before the police advance, AMCU president Joseph Mathunjwa, whose organsiation 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 loud-hailer, 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 percent 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 fell 6.7 percent on Thursday, bringing total losses since the violence began to more than 13 percent. They closed down 7.3 percent in Johannesburg.
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 nearby by Impala Platinum . That helped push the platinum price up 15 percent.
South Africa is home to 80 percent 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.
