"I am following with great sadness the damage caused by heavy rains in (São Paulo)," João Doria wrote on Twitter in a post with photos taken during a flight over the affected region. "My condolences to the families and friends of the 18 people killed," he added.
Five of the dead, from Várzea Paulista, belonged to one family, according to the news website G1. In Embu das Artes, three members of one family died. It has been raining heavily in São Paulo over the past few days. Brazil's most populous state, with more than 40 million people.
Three weeks ago, at least 10 people were killed by landslides and floods in the neighboring state of Minas Gerais.
Inclimate weather may also have led to a tragic accident in which a rock broke off from a cliff and crashed onto a tourist boat on the Furnas reservoir, killing 10 people.
Earlier, the state of Bahia, bordering Minas Gerais to the northeast, had experienced the worst flooding in more than 30 years.
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