VENICE Venice suffered its worst flooding in 22 years on Monday as the 'acqua alta' (high water) stood more than five feet deep before beginning to recede in the Renaissance city.

A change in the direction of the wind helped the water start backing down from a high of 1.56 metres (5 feet, 2 inches), the tide monitoring centre said.

Authorities had warned that the sea lapping at the lagoon city threatened to rise to 1.60 metres, a 30-year high mark, and warned residents and tourists to stay indoors.

'Its an exceptional acqua alta, and unless you absolutely have to, dont go out,' Venice mayor Massimo Cacciari said in a statement.

Nearly all the streets of the city, including the central tourist district, were already under water by mid-morning -- the famous Piazza San Marco by 80 centimetres.

Workers set up elevated walkways as sirens and loudspeaker announcements reinforced the alert. Under a new system, warnings and updates were also being sent out by text message.

The tidal centre predicted earlier that floodwater would fall back to normal levels by 700 pm (1800 GMT), but said another surge was expected in the small hours of Tuesday.

The situation was complicated by a national transport strike affecting the citys 'vaporetto' water bus service.

Experts said the surge in the sea level was caused by a combination of persistently high southerly winds and heavy rain and snowfall in northern Italy over the past few days.

The worst incident of 'acqua alta' in Venice came on November 4, 1966, when the city was submerged by 1.94 metres of water amid catastrophic flooding throughout Italy.

More recently, in February 1986, levels reached 1.58 metres above average.