A violent downpour caused a flash flood late on Saturday that swept up dozens attending a local summer festival in northern Italy, killing four men and injuring about 20 others, rescuers and officials said.

About 100 people had gathered near the banks of a stream in the foothills of the Alps near the town of Refrontolo for the annual gathering when a thunderstorm hit, sending a wall of water and mud into the festival, the fire department said.

"Nobody had ever seen anything like it. There were 2 metres of water. People grabbed onto trees to save themselves," Mirco Lorenzon, a local civil protection official, said in an interview broadcast by SkyTG24 on Sunday.

The storm caused about 50 mudslides and much property damage in the area, Lorenzon said. Pictures and videos posted online showed cars that had been swept away in the flood lying upside-down in the stream or stranded sideways against trees.

"There was an hour-and-a-half of rain so heavy that you couldn't see anything," said Luca Zaia, governor of the Veneto region, during a visit to the site of the disaster on Sunday.

"Within the hour, I will sign papers declaring a state of emergency for the area," Zaia said in a television interview.

Deadly floods and mudslides are not uncommon in Italy. Last November storms in Sardinia caused floods that killed 18 people