Trade union CCOO said that 75.8% of the country's roughly 80,000 banking staff were taking part in the strike.

The leading unions are now demanding a minimum pay rise of 13% over a three-year period from 2024, less than the 17% to 23% increase they previously called for.

Spanish banking association AEB has offered an accumulated 8.25% increase, according to CCOO.

The AEB said in a statement it regretted the strike and the inconvenience it may cause without providing any figures of participation in the action.

"We express our willingness to try to find the necessary meeting points that will allow us to reach an agreement in the next meetings (with unions)," it said.

Spanish banks agreed to raise employees' wages by 4.5% in 2023 compared to 2022, but that only partially offset the loss of purchasing power caused by steep inflation.

While protests in the sector have in the past been mostly directed against staff cuts, attention is now turning to wages.

Banks' recent profits have been buoyed by a rise in interest rates and more expensive mortgages, while keeping a lid on rates for savers.

(Reporting by Jesús Aguado; editing by Emma Pinedo and Mark Potter)