Four people died and about 20 others were injured in flash floods that tore through a village festival in northern Italy, rescuers and officials said.

About 100 people had gathered near the banks of a stream near the town of Refrontolo, north of Venice, for the annual festival when a thunderstorm hit late on Satuday, sending a wall of water and mud into the crowd, according to Italian authorites

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

The storm caused about 50 mudslides and significant property damage in the area, Lorenzon said.

Pictures and videos posted online showed cars that had been swept away in the flood upturned 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," he said in a television interview.

Environment Minister Gian Luca Galletti said the tragedy was a reminder of the need to address the "structural fragility" of the land in some areas.

According to a report by the Institute of Atmospheric Sciences and Climate in Bologna, rainfall in July this year was 74 percent higher than the average for the month between 1971 and 2000.

The number of regions hit by flooding has doubled to eight in the last 10 years, according to Italy's main environmental organisation Legambiente which has called for more money to be spent on prevention.

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