The firefighters were still searching for people trapped inside, and had found none so far, said firefighter spokesman Luca Cari.
The ANSA news agency said around 60 square meters (645 square feet) collapsed from the vault in one of the galleries inside the complex.
Built by Roman emperor Nero in the first century A.D., the sumptuous palace is known to many by its Latin name Domus Aurea.
It was reopened in 1999 following 18 years of closure because of structural concerns, and it soon became one of the capital's most popular tourist sites. But the monument remains plagued by structural problems and water infiltration, which have forced the monuments to shut down at times.
Part of the ceiling already came crashing down a decade ago. It was not clear if it was the same area that collapsed Tuesday.
With its frescoed halls and gold-encrusted ceilings, the palace was meant as a fabled residence for one of Rome's most depraved emperors. It sprawled across nearly 200 acres (80 hectares), occupying parts of four of Rome's seven ancient hills.
But Nero did not enjoy the palace for too long. It was completed in A.D. 68, the same year the emperor committed suicide.
The complex lay under tons of dirt in the bowels of a hill for centuries, before coming to light 500 years ago when Renaissance scholars began researching the imperial period.
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