The Ministry of Health's red alert is the highest possible level, indicating "high risk conditions that persist for three or more consecutive days".
The affected cities are: Bologna, Bolzano, Brescia, Florence, Genoa, Milan, Perugia, Rieti, Turin, Trieste, Venice and Verona.
In all those areas, the red alert will remain in place until at least Sunday.
#estatesicura Prima estesa ondata di calore in gran parte del Paese nel 2018, bollini rossi il 31/07 e il 1/08 in molte città del Nord e del Centro, poi anche del Sud. Bollettini, linee guida e consigli su https://t.co/Mwjz900b2H e app "Caldo e salute". pic.twitter.com/nbbwDKNm9p— Ministero Salute (@MinisteroSalute) July 30, 2018
Unusually, the weather is generally milder in the south, with storms and showers were forecast across the southern coastal regions on Friday.
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"This is the first real wave of high intensity heat extended across most of the country in 2018, which will last for the whole week, also affecting the southern regions," the ministry said.
Many other cities in the country were placed on an orange or level 2 alert. The difference between the two is that while both mean the heat poses a risk to health, in 'level 3' conditions, even healthy and active members of the population are likely to be affected, while during an orange alert, it is predominantly vulnerable groups such as children and the elderly who are at risk.
The government has set up a dedicated helpline, reachable by dialling 1500 in Italy, to offer advice on coping with the hot weather as well as information on healthcare services.