Tourist boat trips down the Tiber were also halted after one of the open-topped boats partly sank during Sunday's downpour.

"For at least the rest of the day, the situation will continue to be one of high alert," a spokesman from Rome's fire service said. Some 400 extra rescue workers have been drafted in from outside the Rome area, Rome newspaper Il Messaggero reported. More than 600 migrants who camp on Rome's outskirts were also at risk, it added.

"We are very concerned, the next downpour is due on Tuesday," civil protection chief Guido Bertolaso said.

While flooding threatens Italy's capital, several rural areas in the central region are already under water. A 70-year-old man had to be rescued by helicopter when his car was washed into a river near Perugia in central Italy during a flash flood.

Italy's agriculture association said in a statement farmers in the Lazio region bordering on Rome and nearby Emilia Romagna, Tuscany and Umbria had suffered tens of millions of euros of damage to their newly harvested grape and olive crops.