Stranded bus passengers try to cross a flooded highway today in Istanbul, Turkey. Flash floods there killed at least 20 people today and stranded dozens. The country has been hit by its worst rain in decades. Since late Monday, at least 28 people have died and more are missing.

ISTANBUL — Flash floods gushed across a major highway and a commercial district in Istanbul today, killing at least 20 people and stranding dozens in cars or on rooftops, the city's governor said. Some of the dead drowned inside their vehicles.

Fueled by the worst rain in decades, waters rose more than three feet high in the city's Ikitelli district, cutting off the route to Istanbul's main airport and the highway to Greece and Bulgaria on the European side of the sprawling city.

Gov. Muammer Guler said eight other people were still missing and 20 others were injured. The deaths raised the overall toll in floods that have slammed northwestern Turkey since late Monday to at least 28.

Stranded motorists desperately climbed onto the roofs of their vehicles, hoping to be rescued. Hikmet Cakmak, Istanbul's deputy governor, described the scene at Ikitelli as a "disaster" and said four helicopters and eight boats were sent to rescue people.

Istanbul firefighters recovered seven bodies at a truck parking lot in Ikitelli, Anatolia reported. Television footage showed trucks crushed against each other by the powerful floods.

Seven other bodies were found outside a textiles factory in the nearby district of Halkali. Guler said the victims were female factory workers who drowned, trapped inside a van that had just brought them to work. Television footage showed seven bodies covered in white sheets, placed in a row.

Guler did not give a breakdown for the six other deaths.

The heavy rains caused two Istanbul streams to overflow, also inundating dozens of homes and workplaces. The state-run Anatolia news agency said one building collapsed, but there were no reports of any casualties.

Rapid population growth — fueled 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.

Istanbul's Ataturk Airport was not affected by the floods, and flights went ahead as normal, said deputy manager Celal Ozugur. However, many passengers could not reach the airport or leave it once they arrived.

A trail of mud covered some areas as waters later receded in parts of Ikitelli. More rains were forecast for northwestern Turkey on Thursday.