They said they were investigating the two arrestees' possible connection with the underworld.
Police said that the two Albanians were in a car on an overpass on central Syngrou Boulevard when the vehicle veered off course and crashed into a protective railing between the two directions of the lanes, resulting in their injury and the immobilisation of the car.
Although injured, one of the Albanians opened the car's boot and threw over the bridge a metal box containing the explosives.
An investigation of the metal box revealed 12 kilos of explosives in gel form.
The two detainees were taken to hospital, where the one was admitted and was under police guard.
The second detainee was led before a prosecutor on Friday, while the first, after his release from hospital, was taken before a prosecutor on Saturday.
Security police believe the explosives were destined for bomb attacks on stores and nightclubs in clearing of accounts between underworld rings.
Both detainees have been referred to an investigating magistrate.