The Civil Protection Department said the Tiber had risen about 16 feet (5 meters) in the past two days and warned it might burst its banks.
Officials evacuated Gypsy camps along the Tiber's banks and boats broke loose from their moorings in the surging water. The smaller Aniene river, which flows into the Tiber, already overflowed, forcing officials to close down some streets in Rome and evacuate hundreds of people.
Tourists snapped pictures as the roiling Tiber surged underneath the city's bridges. Lumir, an Afghan hound, sported a blue raincoat Friday as his owner watched the Tiber rise in Rome.
Italy has been hit by days of bad weather, and TV footage has shown entire neighborhoods flooded or submerged by mud.
Downpours disrupted traffic Friday from Milan in the north to Palermo, Sicily, in the south, as trains were delayed and many streets were flooded or blocked by fallen trees. A few inches (centimeters) of water again covered Venice's lowest parts, including the landmark St. Mark's Square, while Alpine rescuers saved a group of boy scouts who had been trapped on Mount Etna.
Four people were reported killed. Rescuers recovered the body of a man in southern Italy who was swept away in the heavy rains, while an elderly man died after his car was hit by a tree and another one was killed in a car crash in a rainstorm, police in the southern city of Reggio Calabria said.
A woman was killed Thursday after her car was submerged in an underpass in Rome.
In Rome and Venice, two of the hardest-hit cities, union officials called off local transport strikes.
Shows at the Auditorium, an exhibition and concert center in northern Rome designed by architect Renzo Piano, were canceled Friday night.
On Thursday, more rain fell in Rome than the usual average for the entire month of December, city officials said.
On Mount Etna, eight boy scouts were rescued Friday after being trapped by a snowstorm at a refuge on the mountain's north slope at an altitude of 1,700 meters (5,577 feet).
In Venice, alarms sounded early in the morning as the high tide came in and parts of the city flooded. Still, the water was far less than the unusually high tide recorded in the lagoon city last week, when residents and tourists waded through knee-high water, shops were flooded and much of the city was brought to a halt.