Storm Doksuri, a former super typhoon, swept northwards over China after hitting southern Fujian province on Friday, following its battering of the Philippines.
Heavy rains began pummelling the capital and surrounding areas on Saturday, with nearly the average rainfall for the entire month of July dumped on Beijing in just 40 hours.
Swaths of suburban Beijing remain badly hit by the rains -- some of the city's heaviest in years.
On the banks of the Mentougou river, one of the worst affected areas, AFP reporters saw muddy debris strewn across the road.
One man told AFP he had not seen flooding this bad since July 2012, when 79 people were killed and tens of thousands evacuated.
"This time it's much bigger than that," he said, declining to give his name.
"It's a natural disaster, there's nothing you can do," a 20-year-old man surnamed Qi, who was waiting for a taxi with his grandmother outside a hospital, told AFP.
"(We) still have to work hard and rebuild."
On Tuesday, state broadcaster CCTV said that the rains had killed at least 11 people in Beijing, two of whom were workers "killed on duty during rescue and relief".
Thirteen people were still missing, but another 14 had been found safe, the broadcaster said.
In neighbouring Hebei province, nine people were killed and six were missing, it said.
Another two casualties were reported in northeastern Liaoning province over the weekend.
President Xi Jinping on Tuesday called for "every effort" to rescue those "lost or trapped" by the rains.
More than 100,000 people deemed at risk across Beijing have been evacuated, according to state-owned Global Times newspaper.
Authorities have allocated 110 million yuan ($15.4 million) for disaster relief work in the capital and surrounding provinces, CCTV said.
'Endure what we can'
Florist Wang Yongkun, 62, had piled sandbags around the door of his shop, but the floor inside was still coated in mud.
He said in 15 years working there he had never experienced anything like the last few days.
"We started cleaning up in the afternoon yesterday... and woke up again at seven today to continue," he said.
"You just have to deal with it... We will endure what we can."
Further south in Fangshan district, the Dashi River had overflowed, with trees along the riverbank partially submerged, and some sections of the road cordoned off.
Roads were caked in mud, foliage and various debris, including an upturned armchair.
AFP reporters saw collapsed bridges at two locations, with locals saying the damage had happened during the rains.
Earlier social media videos tagged in Fangshan had shown multiple cars being swept along roads turned into fast-flowing streams.
Live images from broadcaster CCTV on Tuesday morning showed a row of buses half submerged in floodwater.
In the parking lot of a high-rise apartment complex, cars were piled on top of each other, alarms still sounding, while people lined up with buckets and other containers to collect fresh water.
Local media on Monday published footage of chaotic scenes aboard high-speed rail trains stranded on tracks for as long as 30 hours, with passengers complaining that they had run out of food and water.
Authorities "must properly relocate affected people, work quickly to repair damaged transportation, communication, and electricity infrastructure, and restore the order of normal production and life as soon as possible", Xi said on Tuesday.
The capital activated a flood control reservoir on Monday for the first time since it was built in 1998, the Beijing Daily said.
Parts of Hebei remain under red alert for rainstorms, with authorities warning of potential flash floods and landslides.
In Handan, Hebei province, rescuers lifted by crane reached a man trapped on top of his car in floodwaters, lifting him to safety before the car was flipped and washed away by the current.
China has been experiencing extreme weather and posting record temperatures this summer, events that scientists say are being exacerbated by climate change.
The country is already preparing for the arrival of typhoon Khanun, the sixth such storm of the year, as it nears China's east coast.