People talk with a police officer along a heavily flooded street in downtown Funchal, Madeira, on Sunday. (Duarte Sa/Reuters)

Emergency crews used sniffer dogs Monday to search through huge piles of debris for at least four people still missing in Madeira after flash floods and rockslides killed 42 people on the Portuguese island.

The dogs scoured heaps of boulders, broken trees and other debris blocking streets. Rescue teams dug cars out of head-high mounds of sludge to see whether there was anyone inside.

Crews pumped rainwater and mud out of a shopping mall's underground parking lot in the island's capital city, Funchal, where officials fear more bodies might be found. The parking lot's two levels were completely submerged.

More than 400 vehicles, including bulldozers and trucks, worked all night Sunday in an effort to clear debris, authorities said.

Several roads remained blocked by boulders, mud and trees that were dislodged by flash floods. The torrent of muddy water swept away people, houses and vehicles on the steep-sloped island. More than 120 people were injured.

People view buried cars in Funchal after the heavy flooding. (Duarte Sa/Reuters)

Light showers were forecast for the Atlantic Ocean island Monday and Tuesday.

Only four people were officially unaccounted for on Monday, but officials said there could be further victims.

The head of the regional government, Alberto Joao Jardim, told people to stay at home Monday and leave their homes only if it was absolutely necessary. Madeira's schools were expected to stay closed, cancelling classes for some 30,000 students.

Locals say Saturday's storm was the worst in living memory. Officials say a month's rain fell on the island over the course of eight hours.

The Portuguese government was to hold a special cabinet meeting Monday and was expected to announce three days of national mourning for the victims.

It may also grant financial aid to rebuild Madeira's many destroyed roads and bridges. The regional government says it has no estimate yet of its financial needs.

Flood knocks down bridges

The victims, in white body bags, were taken to Funchal's international airport, where officials have set up a makeshift morgue.

Madeira, with a population of around 250,000, is the main island of a Portuguese archipelago of the same name in the Atlantic Ocean just over 480 kilometres off the west coast of Morocco.

'There were cars on rooftops ... vans and trucks that had fallen and been totally crushed.' —German tourist Andreas Hoisser

The flash floods were so powerful they carved paths down mountains and ripped through the city, churning under some bridges and tearing others down.

"We heard a very loud noise, like rolling thunder," said Briton Simon Burgbage, who was vacationing on the island. "The ground shook, and then we realized it was water coming down."

Residents caught in the torrent clung to railings to avoid being swept away. Cars were tossed about by the force of the water; the battered shells of overturned vehicles littered the streets.

"It was horrible," said German tourist Andreas Hoisser. "There were cars on rooftops. There were vans and trucks that had fallen and been totally crushed."

A medical team, divers and rescue experts from mainland Portugal, 900 kilometres northeast of Madeira, arrived on the island Sunday aboard a military transport plane.