The Zimbabwean government headed by Robert Mugabe accuses the ZCTU of calling the strike at the behest of the main opposition party, the Movement for Democratic Change (MDC).
Journalists who drove around Harare's industrial areas early on Tuesday found many firms operating as usual, and the normal hordes of job seekers waiting outside factory gates seeking employment in an economy already in freefall.
ZCTU officials were not immediately available for comment, but Lovemore Matombo, the union president, said he hoped workers would risk open defiance to protest against an economic crisis in which inflation has surged to 1,700 per cent, unemployment has reached more than 80 per cent and food and fuel shortages are frequent.
"We are quite aware that the Zimbabwean authorities will never treat us with kid gloves," he told SABC radio in neighbouring South Africa.
"But we are saying the suffering we are going through is even worse than the broken bones that are likely to come our way in the next two days."
The strike comes as Mugabe faces international condemnation over a crackdown last month which left main opposition leader, Morgan Tsvangirai, injured and hospitalised after police stopped a banned prayer rally.
Analysts say the ZCTU's calls for strikes over labour and social issues in recent years have largely failed due to government intimidation and workers' fears of losing their jobs.