Spain's two largest unions, the CCOO and the UGT, led the demonstrations, in Madrid, Barcelona, Vigo and all the provincial capital cities in the Andalusia region, ThinkSpain reported.
The unions say the reform would lead to higher unemployment and reduce consumer spending, including home-buying, putting more companies out of business and making more people rely on government assistance.
The unions also argue the reform would worsen a recession and could lead to job insecurity and high staff turnover, which in turn could result in a "dumbing down" of the workforce as lengthy employment in the same job would be less likely.
Under the reform, some companies may be forced to reduce staff to avoid closing, ThinkSpain said.
Critics of the plan said labor reform could result in some companies firing experienced employees and hiring less-experienced ones at a much lower wage and make it easier for companies to end workers' contracts.
The unions plan a March 29 general strike across the country.