One social media user said residents were stuck on rooftops because they did not know how to swim.
Massive floods in central China caused by record-breaking rainfall have left 12 people dead and more than 100,000 evacuated from their homes.
More than a dozen cities in Henan province, including its provincial capital Zhengzhou, are affected.
Footage circulating online shows people wading in chest-high levels of water on roads and at train stations.
President Xi Jinping said on Wednesday the “flood prevention situation was very severe” and at a “critical stage”.
He added that the floods had already resulted in “significant loss of life and damage to property”, and instructed all departments to prioritise “the safety of people and their property”, according to state news agency Xinhua.
Henan province, home to about 94 million people, has issued its highest level of weather warning. Local authorities called the floods a “once in 100 years” event.
The rain began over the weekend, with storms intensifying on Tuesday.
By Tuesday night, Zhengzhou and other places in Henan province saw widespread flooding. There were reports online of people swept away by fast-moving floodwaters, and cars and debris drifting down submerged streets.
Several dams and reservoirs have breached warning levels, and soldiers have been mobilised to divert rivers which have burst their banks. Flights and train operations in many parts of Henan have also been suspended.
image copyrightGetty Images
Some 700 passengers in Zhengzhou are also said to have been trapped on a train for at least 40 hours, according to local media reports. They were reportedly provided food at first but this is now said to be in low supply.
The First Affiliated Hospital of Zhengzhou Hospital also briefly lost power on Tuesday night, though this has now been restored, said a statement on Weibo by the Zhengzhou Municipal Party Committee.
It added that 600 critically ill patients had been transferred to another location.
A 20-metre (65ft) breach has emerged in the dam in Luoyang city after it was damaged by storms, officials said. Soldiers have been deployed to the area and a statement from the army warned it could “collapse at any time”.
In Zhengzhou, unverified video footage showed passengers in flooded subway carriages with water reaching their shoulders.
Rescuers were seen pulling people to safety by rope, while others stood on train seats to try to keep above the water.
It is not yet known how many people – if any – remain trapped, but hundreds are reported to have been rescued so far.
Many of those affected turned to social media to call for help. One person who went by the name Xiaopei said in a post on microblogging platform Weibo that they were trapped in a train.
“The water in the carriage has reached my chest. I can’t speak any more,” they said. The fire department later confirmed that they had been rescued.
Another user said that residents in Sishui town were stuck on rooftops. “We don’t know how to swim… the whole village is about to be washed away,” the person wrote.
State media also aired footage of a flooded nursery school where children were floated out in plastic tubs by rescuers.
The floods dominated Chinese social media on Wednesday morning, with many on microblogging platform Weibo praying for the safety of those affected.
The hashtags #HenanCanMakeIt and #HenanStormRescue were trending, with close to 1 billion views for each.
Eyewitness account by Zhengzhou resident Mr Liu, 27, as told to BBC Chinese
Last night in Zhengzhou I had no water, no electricity, and no internet. I only regained contact with the outside world this morning.
Never in my life had I seen so much rain.
There was one hour where the rain was just pouring down on us from the heavens, and everything went completely white. We couldn’t see clearly – and then the waters started rising.
Now there’s not much rain. There is a lot of water still on the roads, but it’s slowly subsiding.
Henan has experienced “rare and severe rainfall” since Saturday, China’s meteorological authority said on Wednesday.
Zhengzhou saw 624mm of rainfall on Tuesday, with a third of that amount falling between 16:00 and 17:00 alone, which “smashed historical records”.
It forecasted that parts of the region would continue to see “severe or extremely severe storms” and that the heavy rain would likely only end on Thursday.
The amount of rain over the last three days in Zhengzhou has been reported to be the same as what it would usually receive in an entire year.
Many factors contribute to flooding, but a warming atmosphere caused by climate change makes extreme rainfall more likely.
Part of the Yellow River basin in China, Henan has several major river systems running through the province which are prone to flooding.
Zhengzhou, which has a population of 12 million, sits on the banks of the Yellow River itself.
Scientists have warned that widespread dam construction has exacerbated climate change problems in China’s flood zone, says the BBC’s China correspondent Stephen McDonell.
Connections between rivers and lakes have been cut and disrupted flood plains which once absorbed much of the region’s annual summer downpours.