Dimitris Papaioannou / AP
Onlookers and police gather near the museum in Olympia, Greece, following Friday's theft.
Updated at 10:32 a.m. ET: ATHENS, Greece -- Armed thieves looted a museum on Friday in Greece's Olympia, birthplace of the Olympic Games, stealing bronze and pottery artifacts just weeks after the country's National Gallery was also burgled.
Culture Minister Pavlos Geroulanos offered to resign following the latest robbery.
Police said a female security guard was overpowered before some 70 bronze and pottery objects were stolen. The culprits reportedly used hammers to smash display cases.
"They overwhelmed and gagged the woman who guarded the building," a police official told Reuters. "The value of the objects stolen has not been estimated yet."
Police set up roadblocks in the area as part of a broad search to try to locate the robbers, who wore ski masks.
Sporting authorities are to hold a ceremony at the museum on May 10 to light the Olympic flame for the London Games.
Olympia Mayor Efthimios Kotzas urged authorities to improve security at the site.
"The level of security is indeed lacking," Kotzas told state-run NET television. "These are treasures. A piece of world heritage has been lost thanks to these thieves ... I think (authorities) should have been more mindful and the security should have been more serious."
In January, three works of art, including one by Pablo Picasso and another by Dutch painter Piet Mondrian, were stolen from the country's National Gallery in the capital, Athens.
After the first gallery robbery, culture ministry officials said cuts to museum budgets had damaged security arrangements.
Greece has seen a rise in crime as its debt-laden economy has shrunk 16 percent in size from its 2008 peak, leading to youth unemployment of just under 50 percent.
A government spokesman said Prime Minister Lucas Papademos would decide whether to accept Geroulanos' resignation once all the facts were clear.
According to the FBI, the stolen antiquities trade is worth more than $6 billion per year.
The agency runs the National Stolen Art File, a computerized index of reported stolen art and cultural properties, for the use of law enforcement agencies across the world. It also maintains a task force 13 special agents dedicated to investigating the trade.
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The Associated Press, Reuters and breakingnews.com's David Wyllie contributed to this report.