Thousands of demonstrators have rallied in cities across Argentina to mark a year since a key witness in a landmark human rights trial disappeared.
Protesters fear Mr Lopez has been kidnapped or killed for testifying
Retired bricklayer Jorge Julio Lopez vanished after testifying in the case of ex-police chief Miguel Etchecolatz.
Mr Etchecolatz is now serving a life sentence for human rights abuses committed under military rule in Argentina between 1976 and 1983.
It is feared Mr Lopez was kidnapped or killed for testifying.
Several other key witnesses and three judges said they were threatened for involvement in the same case.
"We're living in democracy and my old man is still disappeared," said Ruben Lopez, son of the missing man, who was 77 when he vanished.
"As a country, we can't allow this."
As thousands of protesters rallied outside the government palace in the capital Buenos Aires, Argentine Interior Minister Anibal Fernandez said the authorities were doing everything possible to find Mr Lopez.
An estimated 30,000 people were kidnapped, tortured and killed during Argentina's "dirty war", when the military government persecuted suspected dissidents.
Under the four-year-old government of President Nestor Kirchner, courts have revived human rights trials against former members of the military and police for decades-old abuses.