Thousands of mine workers downed tools on Tuesday at South Africa's Lonmin mine after a union leader was shot dead in the restive platinum belt at the weekend.
"Lonmin operations are suspended this morning due to an illegal work stoppage," Lonmin spokeswoman Sue Vey told AFP.
Work had stopped at all of the firm's 13 shafts in the northwestern Rustenburg mining town, the world's top platinum producing region, she added.
The reasons for the strike remain unclear but it comes at a time of deadly union rivalry.
By early afternoon around 400 miners had gathered near the hill where 34 of their colleagues were killed in clashes with police last August.
They demanded a rival union be ejected from the mine.
Some of the group chanted and carried sticks and branches within sight of a string of white crosses that serve as a memorial to one of the most bloody incidents of police brutality since the end of apartheid.
A small police contingent stood at a distance.
The strike raises the spectre of further violence between rival unions and another body blow to the struggling South African economy.
Lonmin shares plunged over seven percent around midday on the London Stock Exchange amid the industrial action.
The firm said the reasons for the strike were unclear and officials were locked in talks with worker representatives.
But the strike comes amid deadly tensions between the long-dominant National Union of Mineworkers (NUM) and its now larger rival the Association of Mineworkers and Construction Union (AMCU).
AMCU was recently recognised as the majority union at Lonmin and at neighbouring Anglo American Platinum, dislodging the powerful NUM from the top position.
"We are here to demand the removal of NUM from the mine," said striking worker Mandisi Dlamini.
"They are no longer the majority union but Lonmin is siding with them.
"They are using guns to kills our people who are AMCU members and Lonmin is protecting them," he added.
"If the mine does not give us assurances that NUM will be kicked out the strike will continue."
One miner however said he joined the strike out of fear for his life.
"To be honest I don't understand what is going on now, I was told that today there is no work and we are expected to gather here," a miner named Clive told AFP.
"Here you do as you are told, you don't want to be seen as doing the opposite," he said.
The union unrest has caused several deaths.
An AMCU leader was gunned down on Saturday while he was watching football in a tavern in Rustenburg.
Mawethu Steven was due to testify at a judicial inquiry into last year's Marikana killings.
Twin brothers were also shot dead Saturday at an informal settlement where some of the Lonmin mine workers live.
NUM claim one of the slain is its representative.
A senior NUM official meanwhile alleged the Lonmin striking workers intimidated its representatives.
"There is intimidation and violence. They are singing and dancing and have blocked roads. Cars are being turned away. Its really bad," said Mxhasi Sithethi, a regional NUM coordinator.
The latest stoppage comes at the start of South Africa's so-called strike season when tens of thousands of workers pour onto the streets demanding tough wage hikes.
The tensions continue in the Rustenburg area, where Anglo American Platinum last week announced 6,000 job cuts in restructuring.
Job losses, union rivalry, and police brutality "could lead to the situation ultimately boiling over," according to Nomura bank analyst Peter Attard Montalto.
"All this only goes to reinforce the underlying competitiveness loss of the mining industry in South Africa," he added.
"We think the market underestimates the structural changes, union dynamic and semi-permanent state of output losses and so ongoing economic impact.