By Wendell Roelf

CAPE TOWN, July 9 (Reuters) - Thousands of unionised South African workers downed tools on Wednesday to protest against a sharp jump in food and fuel prices, threatening to slow output at key ports and refineries and rattle jittery investors.

The walkout in the Western Cape and KwaZulu-Natal provinces, home to two of the nation's three largest cities, was part of a drive by the Congress of South African Trade Unions (COSATU) and its allies to pressure the government to tilt policy to the left in Africa's biggest economy.

It will culminate in a national strike on Aug. 6

The mass action came amid rising political tension in South Africa and fears of a downturn in its economy. Growth has averaged about 5 percent in recent years but dropped sharply in the first quarter of 2008.

Business leaders and foreign investors worry that labour unrest could combine with an electricity crisis and soaring world oil prices to produce a toxic cocktail for South Africa, which is a net importer of energy.

"The proposed rolling mass action will affect productivity and drive prices higher. We are looking at millions of rands, if not billions in lost productivity," said Neren Rau, chief executive of the South African Chamber of Commerce and Industry.

Unions, which were instrumental in helping to end white minority rule, accuse President Thabo Mbeki of pursuing pro-business policies that have favoured a wealthy elite at the expense of millions of workers and the poor.

A sharp increase in the price of petrol and basic staples, such as milk and bread, and a series of interest rate hikes have stoked labour's determination to thwart the policies.