"I'm not worrying," although the detachment "is not to be underestimated," Umberto Broccoli, superintendent of cultural heritage for Rome, was quoted as saying by Italy's ANSA news agency.
About 120 square feet of the 2,000-year-old structure were cordoned off Wednesday after bits fell from the highest arcade.
Broccoli said weather extremes this year had weakened part of the Colosseum.
Repair work funded by shoe magnate Diego Della Valle is scheduled to begin in 2013.
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