The man was arrested in the town of Patra, some 200km from the capital.
Police discovered him at a resort close to the bus station in the port metropolis, public tv channel ERT reported.
He was discovered in possession of a second weapon, a loaded .38 calibre revolver, based on native media.
According to native media experiences, the attacker had entered the buildings with a shotgun hid underneath his coat.
State broadcaster ERT confirmed surveillance footage of the suspect calmly leaving the constructing on foot – with police investigating the likelihood he had caught a taxi to the second location.
“I saw the whole thing… he was firing at the ground,” one eyewitness advised state information company ANA.
“Four women working at the small claims court were slightly injured,” the pinnacle of the judiciary workers’ union in Athens, Stratis Dounias, advised the broadcaster.
ERT reported the five individuals – a person and 4 girls – have been left with non-life-threatening accidents in the shootings.
Gun violence in Greece is comparatively unusual, as firearm possession is allowed however closely regulated.
