JOHANNESBURG (Reuters) - South African police fired tear gas and rubber bullets at protesters hurling petrol bombs after a march on an Anglo American Platinum mine turned violent overnight, the latest sign of festering labour unrest.

Police stand guard over a vehicle burnt in strike related violence close to a mine belonging to Anglo American Platinum outside Rustenburg in the North West province, October 12, 2012. REUTERS/Mike Hutchings

Africa's top economy is under increasing pressure to mend industrial relations. Almost 100,000 workers, mostly in mining, have launched often illegal and violent strikes since August, eroding investor confidence and already shaky growth.

Truck drivers signed a wage deal on Friday ending a three-week strike that squeezed deliveries of fuel, cash and consumer goods. But that step forward was quickly overshadowed by a Standard & Poor's downgrade of South Africa's credit rating.

About 1,000 protesters gathered on Friday night in a shanty town near Rustenburg 120 km (70 miles) northwest of Johannesburg and marched toward Amplats' Khomanani 1 shaft, the South African Police Service said in a statement.

"Tear gas and rubber bullet rounds were used to disband the overzealous group," it said.

The crowd responded with petrol bombs, damaging one police vehicle, police said. There were no reports of injuries and four people were arrested.

Amplats, as the company is known, is the world's top producer of platinum. Striking leaders from Amplats and other mines met on Saturday to discuss strategy.

"All of the mines that you know are striking, their (strike) leaders are here," labour leader Evans Ramokga told Reuters. "Right now we are talking about the way forward... We are not afraid of dismissals."

Amplats, which has said it is losing an average of 3,800 ounces of production for each day of the strike, has fired 12,000 wildcat strikers. Other mining firms have followed suit.

More than 50 people have been killed in labour-related unrest in the last two months, including 34 shot dead by police at Lonmin's Marikana platinum mine on August 16 in the deadliest security incident since the end of apartheid.

IMPALA MARCH

Hundreds of miners marched on the headquarters of Impala Platinum in Johannesburg on Saturday to deliver a list of demands to the company.

"We are here, we are at Impala," Lesiba Seshoka, a spokesman for the National Union of Mineworkers (NUM), told Reuters by telephone as miners cheered and chanted in the background.

Seshoka put the number of marchers at 2,000, but the company said it was close to 400. A spokesman for Implats said the union's demands were not wage-related and had to do with other contract issues.

Kumba Iron Ore, one of the world's major producers of the steel-making ingredient, said on Friday it would seek criminal charges against illegal strikers who refuse to stop their occupation of its Sishen mine.

The strikers have seized about 3.3 billion rand (236.4 million pounds) worth of heavy equipment, including 88 haul trucks and 13 pieces of other equipment, including bulldozers.

The weeks of strikes have undermined confidence in South Africa's economy and the government of President Jacob Zuma, which critics say has been slow to respond.

Ratings agency Standard & Poor's cut South Africa's credit rating by one notch on Friday to BBB with a negative outlook, saying the strikes and social tensions could reduce fiscal flexibility and damage growth.

Police on Friday arrested two men for the killing of a NUM branch secretary, which NUM's spokesman has said was done in "execution style". Two workers at different shafts of Lonmin's Marikana mine were arrested, local police spokesman Thulani Ngubane said.

The six-week strike at Lonmin in August and September erupted out of a turf war between the NUM and the more militant Association of Mineworkers and Construction Union (AMCU), which accuses the NUM of selling out workers.

(Additional by Lynette Ndabambi; Editing by Mark Heinrich)

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