The tearful stories swapped outside the Spanish attorney general's office today were of nuns, priests and doctors trafficking in babies, of tiny corpses kept in fridges, of coffins without bodies and of mothers who only caught one fleeting glimpse of their newborn children.

"My baby is not registered as having been born, died or even as being stillborn. She simply does not exist, but I heard her cry," said Ana Páez, 51. She was one of 261 mothers, fathers, siblings and adoptees demanding that authorities investigate whether they were victims of a baby-stealing network in Spanish hospitals.

Páez travelled from Barcelona today to add the daughter she says was born in 1981 at the Vall d'Hebron hospital to the other cases that campaigners want investigated by the courts.

"I want to know where she is," she said. "I have cried for her often and I want her to know that I didn't give her away, that she was stolen. Now I know I am not the only one."

Adela Alonso, 70, said: "The same thing happened to me in Valladolid in 1967. I heard that baby cry, but two days later they said he was dead and had buried him.

"I want him to know that he has two brothers and lots of cousins."

While most had no proof that their children were stolen, their stories had one thing in common – the absolute power exercised by maternity hospitals over their patients, especially poorer ones, for decades.

The fact that hospitals were responsible for burying children who died at childbirth meant the number of potentially suspicious cases was huge. In some cases the paperwork is missing.

"We were told that there is no register, that our case simply does not exist," said Antonio Moreno, who was searching for a sister born at a clinic in Terrassa, near Barcelona, in 1967.

Clandestine networks that once encouraged single women to hand their babies over for secret adoptions by respectable families in a mostly Roman Catholic country have added further confusion to past events.

In those cases, births were sometimes registered fraudulently as being born to the adoptive mother. Lax adoption laws were not changed until 1987.

A Madrid clinic that closed in the 1980s after being investigated for its role in illegal adoptions, the Clinica San Ramón, is at the centre of the allegations. Journalists found a baby's corpse in a fridge, leading to rumours that bodies were kept to show parents who doubted their own child had died. A former clinic employee recently confirmed that babies were illegally given up for adoption.

A group of people born at the San Ramón and adopted by families from the Valencia region are among those now seeking their biological parents.

"The stolen children are looking for their parents and the mothers for their children," said Enrique Vila, the lawyer representing them.

Revelations that, in its early days, Franco's regime also removed children from new mothers deemed to be dangerously leftwing have added to mistrust of the medical system.

Campaigners insisted today that while the theft of babies may have originated in the early days of Franco, it had proved so lucrative that it continued after the dictator died.

Estefania Anguita, 24, wanted to know what happened to her twin sister, Amanda, born at La Alianza clinic in Barcelona in 1986.

"We were both born healthy, but two hours later they said she had died. The hospital said it buried her. There are five more cases from the same hospital."

Antonio Barroso, an adoptee who is leading the campaign, said: "If they refuse to accept the case then we will go to the European court of human rights at Strasbourg, because we are not going to stop."