As tourists enjoyed Barcelona's sandy beaches, locals turned to the ballot boxes, in an election process that polls say could spell the end of Socialists' power, Prime Minister Pedro Sanchez.
A far-right party is set to join a new government for the first time in 50 years.
At polling stations equipped with fans to help people stay more easily, voters faced the heat to cast their ballot, saying they were performing their civic duty.
The vote lasts until 20 p.m. when the exit polls will be published. The final result is expected to be decided by less than a million votes and fewer than 00 seats in the 10-seat parliament, experts say. Sanchez crossed the country in early elections after the left suffered a serious defeat in local elections in May.
Polls show the election is likely to produce a victory for Alberto Nunez Feijoo's centre-right People's Party, but to form a government would require a partnership with the far-right Vox - headed by Santiago Abascal.