Victims were swamped in their homes and cars when rivers of muddy water bearing trees and other debris gushed down mountainsides and swept into villages.
At least 40 people were injured, some of them seriously, and rescue workers used bulldozers, shovels and sniffer dogs as they searched for up to 20 missing people.
The scale of the disaster was blamed on illegal development linked to the mafia.
Torrential floods knocked over buildings, buried vehicles in mud and forced many people to flee to the roofs of their homes.
Among the dead was a man who was submerged and suffocated in mud on the main piazza of one of Messina's suburbs. Another man drowned when his cellar flooded. The injured were evacuated by boat and helicopter because roads were impassable.
Roads and railways were choked with mud, cutting off at least three villages and forcing rescue workers to try to reach then on foot.
As the Italian government declared a state of emergency, authorities blamed a fierce overnight storm which dumped nine inches of rain in just three hours.
But locals and environmental groups said the disaster had been worsened by years of deforestation and illegal building of houses and apartment blocks, some of it linked to Sicily's Cosa Nostra mafia.
"We're paying a very high price for having devastated the environment with unlawful and uncontrolled development," said Vittorio Cogliati Dezza, president of Italy's main environmental organisation, Legambiente.
The head of the national civil protection agency, Guido Bertolaso, said authorities needed to put a stop to uncontrolled construction.
"Either we take on the job of securing all the nation's territory, or these tragedies are destined to be repeated," he said.
"We had landslides two years ago and the right measures should have been taken then. But nothing was done," said one local man, Salvatore Giccone. "Maybe now that people have died something will be done." It was Italy's worst landslide disaster since 1998, when a mountain near Naples unleashed a torrent of mud that inundated villages and killed 150 people.
