Two masked men smashed display cabinets and took more than 60 objects after overpowering a guard at the museum in Olympia, officials said.
The town's mayor said the items, mostly bronze and clay statuettes, were of "incalculable" value.
Culture Minister Pavlos Geroulanos has tendered his resignation, but it has so far not been accepted.
He has gone to the site which is on a forested hilltop in western Greece.
The BBC's Mark Lowen in Athens says the robbery - the second major museum theft this year - raises fresh questions about museum security.....