Spanish police have arrested a man accused of performing more than 100 forced abortions on women fighters with Colombia's largest rebel group, the Farc.
The man, Hector Arboleda Albeidis Buitrago, had been working as a nurse in Madrid, the authorities said.
Colombia is seeking his extradition.
On Friday, Colombia announced it was investigating at least 150 cases of former fighters who said they were made to terminate their pregnancies.
Mr Albeidis Buitrago, known as "The Nurse," has been accused of taking part in most of those abortions.
'Lucky to give birth'
Female rebels were compelled to have abortions so as not to undermine their fighting ability, Attorney General Eduardo Montealegre told reporters on Friday.
"We have evidence to prove that forced abortion was a policy of the Farc that was based on forcing a female fighter to abort so as not to lose her as an instrument of war," he said.
The left-wing rebels have denied this in the past, saying contraception was readily available.
One woman who left the rebel group told the BBC's Natalio Cosoy in Bogota that she had been forced to have five abortions.
Women in the organisation were expected to fight, she told our correspondent. The fighters who were allowed to have babies considered themselves lucky.
Peace talks in Cuba
The Farc has fought a five-decade insurgency in Colombia but peace talks in Cuba have made significant progress since they were launched in November 2012.
Agreement has been reached in four broad areas during three years of talks with the Colombian government.
These include how the justice system deals with crimes committed by the rebels and government forces.
Colombian government negotiators and Farc delegates have said that they are hoping to sign a final peace agreement in March 2016.
If the deal is approved by the Colombian people in a referendum, the rebel group will give up its armed struggle and join the legal political process.
The Farc, which was formed in 1964 with a vow to install a Marxist regime, once controlled a swathe of Colombian territory the size of Switzerland.
But the group has suffered a number of setbacks in recent years and has become increasingly involved in the drug trade.
More than 220,000 people have been killed in the conflict, the majority of them civilians.