Protesters took to the streets waving flags in Madrid riot police were deployed in Barcelona where bins were set alight and shops attacked.
A total of 58 people were detained and nine injured in scuffles as the strike got under way a minute after midnight, the interior ministry said.Hundreds of domestic and European flights were cancelled. Regional TV stations in Andalusia, Catalonia and Madrid were also off the air.
Workers at Renault, Seat, Volkswagen and Ford car factories honoured the strike, the UGT union claimed.
Protesters are challenging measures by conservative prime minister Mariano Rajoy's newly-elected government to make it easier for companies to downsize and cut wages, which it hopes will create jobs in the future.
Protester Angel Andrino, 31, was laid off a day after the labour reforms were approved in a decree last month.
'The rights our parents and grandparents fought for are being wiped away without the public being consulted,' he said.
The government said the strike had failed to cause major disruption. 'There is no stopping on the path to reform,' said labour minister Fatima Banez.