Flooding struck the northern outskirts of Rome on Wednesday after heavy rains in central Italy swelled the Tiber River, as the historic city of Venice and popular tourist regions of Tuscany and Umbria counted the costs of extensive flood damage.

The river that snakes through the Italian capital broke its banks in some areas, tearing boats from their moorings and sweeping debris in the current. The Aniene, a tributary of the Tiber in northern Rome, also flooded.

The flooding snarled traffic in a few areas of Rome as streets and bridges were closed off. A fire brigade helicopter could be seen circling overhead to survey the damage as rescue workers carried out checks.

Sports centres, rowing clubs and tennis courts along the river were covered in brown water and two middle-aged men were plucked from a derelict house on the bank, where many homeless people and undocumented migrants find shelter.

"We lost everything. The worst is that we weren't warned!" Aurora Donati, owner of a dog-pampering centre along the Tiber, told reporters at the scene.

Regional authorities said in a statement that they were monitoring to ensure the higher water did not flood the city's sewer system and warned that the river's swell could last "even several days" and subside only gradually.

They also said the centre would be spared because of its high embankments.

Massive clean-up operations were under way meanwhile in other parts of Italy, particularly in coastal areas of Tuscany and low-lying parts of Umbria where flooding in recent days has hit popular resorts and farming centres.

"Our top priority is making sure people are safe by removing water and mud. I hope we complete this in the next few days," said Tuscany's governor Enrico Rossi, asking for special powers and state funding for reconstruction efforts.

In Venice, the consortium overseeing construction of a barrier to protect the historic city from rising sea levels said the defences were more necessary than ever after waters reached their highest level in four years on Sunday.

"There will be an apocalypse if we do not act fast," said a representative of the Consorzio Venezia Nuova, speaking on condition of anonymity.

The consortium said the MOSE water barriers around the Venice lagoon are three-quarters complete but that an extra 1.5 billion euros ($1.9 billion) in government funding will be needed to complete the project by 2016.

The level of the water in Venice reached 149 centimetres (nearly five feet) on Sunday -- its highest level since 2008 -- sparking a red alert in the city, where hardy bathers were seen taking a dip in the flooded historic St Mark's Square.