Brazilian megalopolis Sao Paulo saw violent storms and heavy rains Wednesday, causing flooding and traffic chaos as authorities announced a state of alert in some parts of the city.

The east of Brazil's richest and largest city, home to some 20 million people, was worst hit as roads became streams carrying vehicles along after the heavy dump, which followed months of the worst drought in decades.

Traffic authorities said some roads were completely flooded, while some shops were badly damaged, although there were no casualties reported.

Cars were overturned and piled up in parts of Sao Paulo, where streets were clogged with trees, branches and other debris.

Workers headed out into the city to clear mud that caked most of the city's roads after it was lashed by the storm.

Residents were warned to prepare for further flooding, although the rains became less intense by late afternoon.

With Sao Paulo suffering major tailbacks even under normal conditions, traffic was badly affected while G1 web news portal said some metro services were running at reduced speed as a precaution.

As commuters suffered, there was a silver lining for the city in that the rains brought some relief after the drought had left reservoir water supplies running dangerously low.

"The rains of today won't turn the water crisis around but they will help," said meteorologist Michael Pantera.

Supplies have been hampered not just by a lack of recent rain but by poor storage infrastructure and poor management of supplies, experts say.

Although state governor Geraldo Alckmin has insisted there is no need for rationing, some municipalities where Sao Paulo's main water company does not operate have had to cut back supplies in recent months.