The general strike on Wednesday — Spain's first in 14 years — is causing major delays in the public transport system and slowing down factories across the eurozone's fourth-largest economy.
Many schools and universities reportedly closed or are working at minimal capacity.
In the capital, Madrid, this adds to an ongoing strike started by teachers nearly a month ago to demand better working conditions.
In Barcelona, the country's second-largest city, public transport was at a standstill or facing long delays in the morning rush hour.
Unions have called for Madrid workers to gather and march through the city centre later on Wednesday.
Marches and gatherings are also planned in Barcelona and other cities.
The two main unions calling for the strike, UGT and CCOO, said in a statement there was widespread participation "with significant work stoppages in strategic sectors".
The unions claimed 90 per cent involvement in the automotive and port sectors, 85 per cent in the meat industry, 82 per cent in waste collection and 75 per cent in the energy sector.
"It's extremely important to remind people that we, the workers — who are also renters or who also pay mortgages — are the ones with the power to halt production and bring the economy to a standstill," Tenants' Union spokeswoman Sandra Mora told the national radio, RNE.
Minor incidents were reported in the first hours of the strike, including roadblocks — some with burning tires — in Barcelona.
There were also scuffles between picketers and police near the country's main wholesale market on Madrid's outskirts.
Public anger over housing reached a boiling point following the nationally televised eviction of an 87-year-old woman from her home in Madrid in October.
The subsequent death of Maricarmen, as the woman became known, caused even more indignation and solidified her as a symbol of Spaniards who feel they pay too much in rent and can't even dream of home ownership.
