The unions are demanding the government drop plans to sell off stakes in state-run companies and change labor laws.
Response to the strike has been mixed. Some banks are closed and public transport has been disrupted.
Reports say some 150 million workers - mainly in banking, manufacturing, construction and coal mining industries - belonging to 10 major unions are expected to stay away from work on Wednesday.
The strike appears to have hit public transport, with long queues of commuters and school children seen at a bus stops in many cities, including the capital, Delhi. Many passengers were also stranded at railway stations and airports.