Storm Alex has left at least two people dead and 30 missing in southern France and northern Italy, bringing record rainfall in places and causing heavy flooding that swept away roads, damaged homes and left thousands without power.

In Italy, at least two people died – a volunteer firefighter in the Aosta Valley and a man in his 30s whose car was swept into the River Sesia, after a road subsided. Twenty-two people were also missing, including six German trekkers who failed to return from a trip in the mountains in the province of Cuneo.

The water level in the River Po jumped by three meters in just 24 hours and officials in the Piedmont region reported a record 630 millimeters of rain in just 24 hours in Sambughetto, close to Italy's border with Switzerland. 

Piedmont regional chief Alberto Cirio called on the government to declare a state of emergency. Dozens of firefighters were trying to reach one village by train after the road was shut, while neighboring Switzerland was also battered with record rainfall and powerful gusts, forcing the closure of roads and mountains.

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At least eight people were missing in France, authorities said. These included two firemen whose vehicle was carried away by a swollen river, according to local witnesses cited by several French media.

The storm ravaged several villages around the city of Nice on the French Riviera. Nice mayor Christian Estrosi called it the worst flooding disaster in the area for more than a century, after flying over the area by helicopter. "The roads and about 100 houses were swept away or partially destroyed," he told French news channel BFM.

"I do not hide from you our deep concern about the final outcome of this episode," said prime minister Jean Castex, adding "I have been particularly shocked by what I saw today." Amid concerns that the death toll could rise, Castex said that the government had triggered its emergency plan for handling natural disasters.

Eric Ciotti, a member of French parliament who is from one of the worst affected villages in the area, Saint-Martin-Vésubie, said several villages were cut off in the mountainous region's steep-sided valleys. "The situation is catastrophic in some communes," Ciotti said.

Meteo France said that 500 millimeters of rain was registered over 24 hours in Saint-Martin-Vésubie, with close to 400 millimeters falling in several other towns – the equivalent of more than three months of rain at this time of the year.

Authorities in the southern Alpes-Maritimes region had been placed on alert Friday and around 12,000 people in three valleys to the north of Nice were without power early Saturday afternoon.

"We are thunderstruck. We saw the [river] Vesubie burst its banks - everything was swept away, including part of the old iron bridge," said Serge Franco, a resident of Roquebilliere.

"My house is habitable but half of my land has been swept away," said another resident, Guillaume Andre, who was evacuated overnight but returned to see the devastation after daybreak.

Director of civil protection Jérémy Crunchant said there was heavier rainfall than on 3 October 2015, when floods caused the death of 20 people in and around the French Riviera city of Cannes.

Meanwhile in Venice, a long-delayed flood barrier system including a network of 78 artificial dykes successfully protected the lagoon city from a high tide for the first time on Saturday, bringing huge relief following years of repeated inundations.