Mugabe's Zanu-PF party pushed the bill through parliament last September despite fierce resistance from the opposition Movement for Democratic Change (MDC).
The government says the bill is part of its drive to empower the country's poor majority, but the MDC complains that it will enrich a few powerful individuals while winning votes in the March 29 elections.
The government has sought to allay business fears of a blanket seizure of companies by saying authorities would work with businesses to set timetables for foreign-owned firms to transfer shares to locals.
Loans will be provided to Zimbabweans intending to acquire shares, start businesses or expand existing ventures.
But analysts say the move could further harm an economy that has already suffered because of foreign investor flight and an inflation rate of more than 100,000 per cent.
Multinational firms that could be affected by the new law include Barclays Bank, Bindura Nickel Corporation and the mining giant Rio Zim.