A 3.5 magnitude tremor struck Siena shortly before 10pm local time on Wednesday, but no damage was reported.
That was followed by some 20 smaller tremors overnight of around 2.0 magnitude, and several more on Thursday morning, according to real-time earthquake reports from Italy's National Institute of Geophysics and Volcanology (INGV).
Siena's city council said on Thursday that Mayor Luigi De Mossi had closed "the city's schools and museums... as well as the universities".
MAP: Which parts of Italy have the highest risk of earthquakes?
The southern Tuscan city is famous for its artistic heritage and for the Palio, a spectacular horse race that pits neighbourhoods against each other twice a year, on July 2 and August 16.
Its historic centre is listed as a World Heritage Site by UNESCO.
Italy is among the European countries most prone to earthquakes. Deadly quakes in 2009 and 2016 killed approximately 300 people each.
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