Describing it as an "unacceptable practice", Spanish Foreign Minister Jose Manuel Garcia-Margallo said if it was discovered the US had spied on Spanish citizens, "it could mean a rupture in the traditional climate of confidence" between the two countries.
Spain would protect itself "at the maximum level" with the country's penal code and could open legal action against the US National Security Agency (NSA), he said.
The minister made the remarks after a 40-minute meeting with the US ambassador to Spain James Costos and the Spanish Secretary of State for the European Union Inigo Mendez de Vigo at the offices of the foreign ministry.
Spain: US spying on Spain an unacceptable practise
The meeting came on the day when former Guardian journalist, Glenn Greenwald, the man who helped former CIA contractor Edward Snowden leak documents on the extent of US spying, published an article in Spain's El Mundo newspaper. It alleged that NSA had spied on over 60 million telephone calls in Spain between Dec 10, 2012 and Jan 8, 2013.
The document, entitled 'Spain last 30 days', reportedly mentioned the calls and although it did not give details of their contents, it did give details of the numbers used, the places the calls were made from and the durations of the conversations.
The espionage exercise also included intrusions into personal information through internet navigators which were used to access private e-mail accounts as well as accounts in the social networking sites, Twitter and Facebook.