A rescue worker arrives at a partially submerged vehicle after heavy rain in Istanbul on Wednesday triggered flash flooding. (Murad Sezer/Reuters)

Flash floods gushed across an Istanbul arterial road on Wednesday, killing 20 people and stranding dozens of vehicles, local media reports said.

As waters rose more than a metre high in the city's Ikitelli district, motorists climbed on roofs of their vehicles waiting to be rescued. The floods occurred in the early hours as people began making their way to work, washing over a main road linking the city to an industrial area, an airport and a highway to Greece.

Istanbul Gov. Muammer Guler told The Associated Press that 20 people died in the floods.

Seven bodies were recovered outside a textiles factory in the district of Halkali, the state-run Anatolia news agency and other news reports said. Anatolia said the victims — all factory workers — were swept away by gushing waters as they left a van that had just brought them to work.

CNN-Turk television said seven other bodies were recovered at a gas station in Halkali. How the remainder of the victims died is unclear.

The heavy rains caused two Istanbul streams to swell and overflow, also inundating dozens of homes and workplaces.

More people missing

Rapid population growth — fuelled by decades of emigration from Turkey's impoverished rural regions — has meant that the metropolis of 15 million has developed without adequate infrastructure and poor city planning.

"We are saddened by the loss of lives. There are still some people missing and we are searching for them," said Procurement Minister Mustafa Demir, who visited the area. "There is huge damage to infrastructure."

"We need to be more careful when designing infrastructure and cities," he said.

Hikmet Cakmak, Istanbul's deputy governor, described the scene at Ikitelli as a "disaster" and said four helicopters and eight boats were sent to help stranded motorists.

Private Dogan news agency footage showed a man stranded atop an overturned van, then letting himself go in the water and swimming toward four men who catch him and help him out of the flood.

CNN-Turk television showed rescuers in an inflatable boat assisting another man stranded inside a van.

Rain in forecast

Authorities blocked off roads leading to the highway and people were being advised to avoid travelling in the area.

Waters receded in parts of Ikitelli later on Wednesday leaving a trail of mud.

Meteorologists said the rainfall that hit Istanbul's Catalca suburb — where two people died on Tuesday — was the worst in 80 years. Six people were also swept away by floods in Saray, in Tekirdag province. Guler said eight other people were missing.

More rains were forecast for northwestern Turkey on Thursday.

Authorities are taking the necessary precautions to deal with the heavy downpours that are expected to continue throughout the week, Guler said.