JOHANNESBURG (Reuters) - Police and protesters clashed overnight near an Anglo American Platinum shaft, South African police said on Saturday, as crippling labor unrest shows no sign of ending.

Police said in a statement officers used teargas and rubber bullets to disperse about 1,000 protesters who were marching on Friday night toward the Amplats' Khomanani 1 shaft near Rustenburg, about 120 km northwest of Johannesburg.

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

Large parts of the platinum and gold mining industry have been brought to a standstill by wildcat strikes that started in early August and intensified after the police killing of 34 strikers at Lonmin's Marikana platinum mine on August 16.

Hundreds of miners marched on the headquarters of Impala Platinum in Johannesburg on Saturday to deliver a list of grievances 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 fellow miners cheered and chanted in the background.

Seshoka put the number of marchers at 2,000, but national broadcaster SABC said the number was closer to 600.

The weeks of strikes have undermined confidence in Africa's biggest 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 hurt growth.

(Reporting by David Dolan; Editing by Myra MacDonald)