Since the end of January, thousands of farmers and livestock breeders have joined a growing wave of protest over low prices and rising production costs which unions say have eroded profitability across the sector.
During the late morning, thousands of demonstrators and hundreds of tractors brought the coastal city of Valencia to a halt, with tractors also blocking many roads in the southern Andalusia region.
Other rallies were taking place in Asturias in the north as well as in the northeastern region of Catalonia.
Unions blame a combination of factors, pointing the finger at big distributors for putting unnecessary pressure on prices and leaving producers with a pittance; cheap competition from outside the European Union; and the government's move to raise the minimum wage.
Deputy prime minister Pablo Iglesias, who is responsible for social rights, and Labour Minister Yolanda Diaz were to meet with union representatives on Friday.
Agriculture Minister Luis Planas on Thursday met with low-cost German discounter Lidl and was to hold talks later on Friday with its low-cost Spanish rival Mercadona which has been accused of choking producers with its pressure on prices.
Unions say the worst-hit sectors are those growing fruit and vegetables, but olive producers have been struggling since October with US tariffs. Spain is the world's top producer of olive oil.