The rallies are the unions' first trial of strength before a general strike called for March 29 to oppose the recently approved reforms and austerity measures.
Most gatherings were preceded by remembrance ceremonies that marked the eighth anniversary of the March 11, 2004 bombings that killed 191 people on Madrid's commuter rail system in Europe's worst Islamic terror attack.
The reforms, passed by decree last month and confirmed in Parliament Thursday, slash the cost of firing workers and ease conditions under which they can be dismissed.
"This is a reform that is not going to create jobs, so it is not justified," said demonstrator Marta Lois, 39, adding, "they are cutting our rights like never before."
The leaders of Workers Commissions and the General Workers Union, who jointly called the stoppage and Sunday's rallies, met before a large march in Madrid to call on the government to negotiate with them over the introduction of what they called "drastic" reforms.
Ignacio Fernandez Toxo of Workers Commission said the austerity package was so heavily tilted in favour of businesses that it made working conditions in Spain resemble those that existed under the dictatorship of Gen. Francisco Franco, who ruled from 1939-1975.
"The labour reforms are going to provoke a profound social regression in our country," said GWU leader Candido Mendez.
Milder austerity measures introduced by the former Socialist government provoked a general strike on Sept. 29, 2010, although that stoppage was only partially observed by workers.
Before approving the current reforms, Prime Minister Mariano Rajoy was overheard telling European Union colleagues in Brussels that the measures he had in mind would cost him a general strike.
Rajoy's labour market overhaul is aimed at revitalizing an ailing economy suffering the Eurozone's highest unemployment rate of near 23 per cent.
Rajoy has acknowledged the jobless rate is likely to rise beyond 24 per cent this year despite the reforms, a point taken up by the unions, which argue that what is needed is a jobs package based on fiscal and financial incentives and a crackdown on fraud.
Spain's economy deflated in 2008 with the collapse of a real estate bubble that had previously boosted Spain into becoming the EU's top job creator for a decade.
Rajoy is determined to prove to investors that Spain will not require a bailout like Greece, Ireland and Portugal.