South Africa’s biggest labor organization urged workers to strike on Wednesday to protest what it called ‘‘the cancer of corruption’’ spreading among business and government leaders and threatening the nation’s democracy under President Jacob Zuma.

The action is the boldest step yet by the Congress of South African Trade Unions to pile pressure on Zuma, whom it helped to win control of the ruling African National Congress in 2007 and then turned against after he fired Pravin Gordhan as finance minister in March. That move prompted S&P Global Ratings and Fitch Ratings Ltd. to downgrade the nation’s credit assessment to junk.

‘‘This strike is about sending a message to both government and private sector that as workers and citizens we are tired of corruption,’’ Bheki Ntshalintshali, general secretary of the 1.7 million-member Cosatu, told reporters in Johannesburg on Tuesday. At least 13 marches will take place across the nine provinces.

Cosatu is taking to the streets just three months before the ANC is scheduled to elect a new leader to replace Zuma, 75.