The main quake was recorded at 1:25 am (0025 GMT) at a depth of 2.5 kilometres, according to the National Institute of Geophysics and Volcanology (INGV).
It was followed by a series of smaller tremors, a so-called "seismic swarm".
Photos and videos posted on social media show cars covered in stones and debris, cracked houses and panicked residents pouring into the streets in the middle of the night.
On person was injured by the collapse of a false ceiling, the civil protection agency said, which also reported minor damage.
The fire service said the injury was not serious, adding that it was carrying out checks across the areas of Bacoli, Bagnoli and Pozzuoli.
In Bagnoli, a seaside district in western Naples, schools were ordered to close on Thursday by city authorities, who said waiting areas had also been set up for residents afraid to go home.
Prime Minister Giorgia Meloni said she was constantly monitoring the situation and was in contact with the relevant officials.
Seismic activity is nothing new in the Campi Flegrei (Phlegraean Fields) area, Europe's largest active caldera -- the hollow left after an eruption.
But many of the 500,000 inhabitants living in the danger zone had already been spooked by a 4.4-magnitude quake in May 2024, which was the biggest for 40 years.
At that time, there were no injuries or any major structural damage.
The eruption of Campi Flegrei 40,000 years ago was the most powerful in the Mediterranean.
A resurgence of seismic activity in the early 1980s led to a mass evacuation which reduced Pozzuoli to a ghost town.
Specialists, however, say a full-blown eruption in the near future remains unlikely.
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