"Families' lives were being ruined and we were acting as executioners," David Ormaechea, president of the Locksmiths Union, told AFP. "It was causing us tension and unease."
A wave of evictions of mortgage-holders ruined by the recession has prompted several suicides and sparked a protest movement that last week brought a motion to parliament for a law to end the procedure.
With the locksmiths refusing to take part, some authorities have been asking the fire service to step in and break open the doors of those resisting
On Tuesday in the northwestern city of A Coruña, firefighters were called to help evict an 85-year-old woman who had defaulted on her rent.
A crowd of protestors gathered outside the apartment to block the eviction.
When the firefighters arrived they refused to open the door and some of them joined in the protest.
Firefighters in other regions such as Catalonia and Madrid have since followed their example.
"We come to the aid of people in emergencies. It is contradictory to help the banks that are putting people's lives in danger" by evicting them, Antonio del Rio, a labour union representative for the Catalonia fire service, told AFP.
"The only thing we do is help citizens," said another Madrid fireman, Pedro Campos.
"We only enter a home when there is danger inside. Getting a woman of 85 out of her home is not a situation of danger."
PAH, the campaign movement that brought the proposed law to parliament, says hundreds of thousands of people face eviction following the collapse of Spain's housing boom in 2008.
The resulting recession has driven the unemployment rate over 26 percent, leaving many unable to pay mortgages on houses that have lost much of their value.
Regularly demonstrating on evictees' doorsteps, PAH says it has blocked half a million evictions since 2009, in some cases enabling families to stay
in their homes and pay rent.
Its bill, backed by a petition with 1.4 million signatures, proposes to end evictions and let insolvent homeowners write off their debts by surrendering their home.
Under the current law, a bank can pursue a borrower for the remaining balance of a loan if the value of the seized property does not cover it.