Four facilities in Barcelona were being searched, a Civil Guard official in the city said under rules barring his name from being published.
Spain allows abortion in cases of rape, fetal deformation or danger to a pregnant woman's physical or mental health. In the latter case, this danger must be certified by a doctor other than the one who would carry out the abortion.
The Civil Guard official declined to say exactly what the clinics were suspected of doing illegally.
The raids were ordered as part of an investigation that began following a complaint by a pro-life group called E-Cristians, the official said.
That complaint was filed in January, after Danish television broadcast a documentary in which the gynecologist who runs the four raided clinics, Carlos Morin, was filmed offering to perform an abortion on a female journalist posing as being nearly seven months pregnant, said Pablo Molins, a lawyer for E-Cristians.
The doctor dispensed with the medical-certificate requirement and offered her a form in which she could state she suffered from a grave mental disorder, Molins said.
Morin was among those arrested Monday, the Civil Guard official said.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969