The break happened at the maximum security Korydallos Prison at 6:25 p.m. Sunday, when the helicopter landed in the prison yard. Guards believed the craft was delivering a senior official, but two prisoners scampered aboard and the craft took off, the Reporter.gr Web site said.
The fugitives were identified as Vassilis Palaiokostas, 40 who was serving a 25-year sentence kidnapping and Albanian Alket Rizai, 32 who was serving a life sentence for manslaughter.
Initial reports said one of the two accomplices who commandeered the helicopter and its pilot was Nikos Palaiocostas, Greece's most wanted robber who escaped from the same prison in 1990, Kathimerini reported.
Justice Minister Anastassis Papaligouras ordered an investigation into the jail break.