* Henan province hit by heavy rains since weekend
* Provincial capital Zhengzhou, dozen cities flooded
* Numerous rivers swollen, some reservoirs breached (Adds details of flooding in Henan's capital of Zhengzhou)
BEIJING, July 21 (Reuters) - Large swathes of China's central Henan province were under water on Wednesday, with its capital Zhengzhou hardest-hit after being drenched by what weather forecasters said was the highest rainfall in 1,000 years.
In Zhengzhou, a city of over 12 million on the banks of the Yellow River, 12 people have died so far amid the floods, and about 100,000 people have been evacuated to safe zones, the official Xinhua agency reported, citing the local government.
The lives of millions of people in Henan have been upended since the weekend in an unusually active rainy season that has led to the rapid rise of a number of rivers in the Yellow River basin.
Streets in a dozen cities have been flooded, while dozens of water reservoirs and dams breached warning levels.
Overnight, local authorities said the rainfall had caused a 20-metre breach in the Yihetan dam in the city of Luoyang west of Zhengzhou, and that the dam "could collapse at any time".
Zhengzhou's flood control headquarters said the city's Guojiazui reservoir had been breached.
"Some rivers have exceeded monitoring levels, some dams have broken down, while some railway services have stopped and flights cancelled, causing heavy casualties and property losses," President Xi Jinping said on Wednesday, addressing the situation in a statement broadcast by state television.
"Flood prevention efforts have become very difficult," Xi said.
From the evening of Saturday until late Tuesday, 617.1 millimetres (mm) of rain had drenched Zhengzhou - almost on par with the annual average of 640.8 mm.
The level of rainfall in Zhengzhou witnessed over the three days was one seen only "once in a thousand years", local media cited meteorologists as saying.
Dramatic video shared on social media since Tuesday showed commuters waist-deep in murky floodwaters on a lightless subway train and an underground station turned into a large, churning pool.
On Wednesday, Zhengzhou's transportation system remained paralysed, with schools and hospitals cut off by severe waterlogging, according to state media.
Some children have been trapped in their kindergartens since Tuesday.
The First Affiliated Hospital of Zhengzhou - a vast treatment centre with over 7,000 beds - has lost all power, and even backup supplies were down, the People's Daily reported on Wednesday.
The hospital was racing to find transport to relocate about 600 critically ill patients, according to the report.
Of Henan's 4,098 rainfall measuring stations, 606 have registered more than 250 mm of precipitation since the weekend, with heavy rains expected to persist through Wednesday, the province's chief weather forecaster told domestic media.
Both Henan province and Zhengzhou municipal meteorological bureaus have raised the emergency response for the disaster to level I.
Rain is expected to continue across the province for the next three days, weather forecasts said on Wednesday.
Zhengzhou airport said on Tuesday evening it would temporarily suspend subways and inter-city trains heading to and from the airport, and it would not accept incoming flights from 8 p.m. local time (1200 GMT) on Tuesday to 12 p.m. on Wednesday. (Reporting by Sameer Manekar in Bengaluru, Josh Horwitz and Jing Wang in Shanghai, and Stella Qiu, Roxanne Liu and Ryan Woo in Beijing; Editing by Marguerita Choy and Jacqueline Wong)
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