Hackers attacked the IT systems behind COVID-19 vaccination appointments for the Lazio region.
Hackers have targeted an official website for booking COVID-19 vaccinations, Italian authorities have said.
The website of Lazio, the region of Rome, was unavailable for several hours on Sunday, the municipality confirmed on social media.
The "powerful" cyberattack prevented citizens from booking appointments for a coronavirus vaccine, as well as other services.
According to sources involved in this case, the site was attacked by "ransomware" hackers.
"All defence and verification operations are underway to avoid the possibility of the cut in services," the region said on Facebook.
"The operations related to vaccinations may be slowed down. We apologise for the inconvenience beyond our control."
Prosecutors have opened an investigation into the incident, but authorities have said that vaccination programme in Lazio will "continue as normal".
"There has been no interruption and there will not be in the coming days," the region added in a later statement, "we will not stop in the face of this attack".
The President of the region, Nicola Zingaretti, reported that Lazio had still vaccinated 70% of its adult population on Sunday, despite the cyberattack.
"Thanks to the citizens of Lazio for the cooperation they have shown since the beginning of the pandemic," Zingaretti said on Facebook.
"With pride and determination let's not stop," he added.
In mid-June, the Italian government decided to create a government agency specialised in cybersecurity to fight against the increasing number of computer attacks.
Italy recently announced that proof of vaccination or immunity from COVID-19 would become mandatory to visit restaurants and cultural centres, a move that triggered a series of protests across the country.