About 50 demonstrators protested outside the court as Eduardo Vela, who worked as a gynecologist at the now-defunct San Ramon clinic in Madrid, arrived in court.
Some carried signs that read "Justice!" and "Human rights for stolen babies."
Vela is accused of having in 1969 taken Ines Madrigal, now aged 49, from her biological mother, and given her to another woman who raised her and was falsely certified as her birth mother.
Prosecutors are seeking an 11-year jail term for falsifying official documents, illegal adoption, unlawful detention and certifying a non-existent birth.
In a dark and often overlooked chapter of General Francisco Franco's 1939-75 dictatorship, the newborns of some left-wing opponents of the regime, or unmarried or poor couples, were removed from their mothers and adopted and the practice was later expanded.
New mothers were frequently told their babies had died suddenly within hours of birth and the hospital had taken care of their burials when in fact they were given or sold to another family.
Madrigal, a railway worker who heads the Murcia branch of the SOS Stolen Babies association, said she did not expect Vela would provide answers about her origins or apology.
But she hoped his two-day trial would mark a turning point that leads the authorities to reopen investigations into other "stolen babies" cases.
"Mothers want to tell their children that they did not abandon them... but above all they want to know that they are well."
The practice began after Franco came to power after Spain's 1936-39 civil war pitting left-wing Republicans against conservative Nationalists loyal to the general who was seeking to purge Spain of Marxist influence.
Beginning in the 1950s it was expanded to remove new newborns from poor families as well as illegitimate babies.