The arrests, five in Germany and three in Sweden, represent the latest attempt to pursue justice for the victims of abuses committed in Syria's civil war.
The suspects detained on Wednesday are accused of taking part in a "violent crackdown on a peaceful anti-government protest" in the Al-Yarmouk district in Damascus on July 13, 2012, Germany's Federal Public Prosecutor's Office said.
It said the four stateless Syrian Palestinians and Syrian national detained in Germany were "strongly suspected of killing and attempting to kill civilians, qualified as crimes against humanity and war crimes".
It named the Syrian Palestinians as Jihad A., Mahmoud A., Sameer S. and Wael S. and said that they were part of a pro-Assad militia called the "Free Palestine Movement" (FPM).
The Syrian national, identified as Mazhar J., is believed to have worked for Syrian military intelligence.
"They and other accessories specifically targeted the civilian protesters, shooting at them", resulting in six deaths and other serious injuries, the prosecutor said.
The war between Assad's troops and armed opposition groups, including Islamic State, erupted after the government repressed peaceful pro-democracy protests in 2011.
It has killed more than half a million people, forced millions to flee their homes and ravaged Syria's economy as well as its infrastructure.
Wednesday's arrests took place as a result of work carried out by an investigation team named "Caesar" after a defector who worked as a photographer for Syrian military police.
In 2013 he smuggled more than 50,000 photographs out of Syria, many of them documenting the deaths of prisoners in detention centres or military hospitals.
'Severe and repeated' abuse
German prosecutors said that those arrested in Sweden also belonged to the FPM and participated in the crimes on July 13, 2012.
Ulrika Bentelius Egelrud, the Swedish prosecutor in charge of the investigation, said the suspects were arrested thanks to "good cooperation with Germany, Eurojust and Europol".
German prosecutors say the four Syrian Palestinians also "physically abused civilians from Al Yarmouk severely and repeatedly" between mid-2012 and 2014, including at militia checkpoints on the outskirts of the district, inhabited predominantly by Palestinians.
Mahmoud A. is accused of turning over one individual to Syrian military intelligence to be incarcerated and tortured.
Prosecutors also say that he threatened a woman at a checkpoint with rape and that he forced her "to pay with family jewels for the release of her minor son".
Three people arrested at a checkpoint in Yarmouk and turned over to military intelligence by Mahmoud A. and others were allegedly killed as part of "a scheduled mass execution" in April 2013.
Germany let in hundreds of thousands of Syrians during the 2015-16 refugee influx and has arrested several Syrians since on its soil over crimes committed in their country.
It has used the principle of universal jurisdiction, which allows the prosecution of certain serious crimes -- regardless of where they took place -- to try Syrians over atrocities committed during the country's civil war.
One of the most high profile cases to be brought to trial was that of a former Syrian colonel who was found guilty in January 2022 of crimes against humanity committed in Damascus.
In recent years there have also been several investigations in Germany, Austria, Norway, France and Sweden targeting people suspected of crimes in Syria's civil war -- particularly those committed by pro-Assad forces.
Last month a Swedish court acquitted a Syrian former general of war crimes charges, saying prosecutors had not proved his involvement in the army's "indiscriminate attacks".
Former brigadier general Mohammed Hamo, 65, was one of the highest-ranking Syrian military officials to stand trial in Europe.
Sweden was the first country to sentence a former Syrian soldier for war crimes in 2017.
