A former worker for Sweden's security police and armed forces went on trial with his brother on Friday, both accused of espionage on behalf of Russia.
Prosecutors earlier this month said the indictment accuses the pair of actions directed against Sweden's intelligence and security system.
The men, aged 42 and 35, were detained last year and have previously denied all allegations.
They are suspected of providing Russian intelligence agency GRU with classified information for a decade. The older brother is also charged with gross unauthorised handling of secret information, according to the prosecution.
Anton Strand, lawyer for the eldest man, said on Thursday his client continues to deny the accusations.
"He has been working in different positions for the Swedish government and has always done his best to do a good job in a very difficult environment," Strand said.
The lawyer for the second brother was not immediately available for comment on Thursday.
Sweden, for centuries a non-aligned nation, hopes to soon join the Western military NATO alliance amid rising security concerns following Russia's invasion of Ukraine.
The trial is expected to mostly take place behind closed door due to national security concerns. It is expected to run until at least Dec. 12.
In a separate case, Swedish police this week arrested two people on suspicion of espionage. One is suspected of gross unlawful intelligence activity against Sweden as well as another country.
Police did not say whether the suspects were Swedish or foreign nationals or say who they were alleged to be spying for.
(Reporting by Anna Ringstrom and Marie Mannes)
