BEIJING, July 21 (Xinhua) -- The heaviest rain in 61 years that lashed Beijing Saturday have left at least four people dead and six others injured, local authorities said.

Two people were killed and six others injured as strong winds toppled rooftops in two villages in the city's suburban Tongzhou district, the Beijing Emergency Medical Center said.

The third person, also in Tongzhou, was killed after being struck by lightning. The fourth, head of a police station in suburban Fangshan district died of electric shock from a fallen electric wire in waters while rescuing trapped villagers.

A driver was rushed to hospital after being pulled critically injured out of her car submerged in deep waters under the Guangqumen overpass. Her fate was not immediately known.

Many passersby have joined the efforts to help drag more cars trapped out of the deep waters.

In addition, more than 5,200 residents in several mountainous suburban districts have been relocated to safer places as authorities ordered to brace for potential landslides and flash flood.

Also rescuers have reached the 350 students and 40 teachers trapped at a military training site in Fangshan district by a flash flood.

The students and teachers were in no immediate danger at higher floors and the rescuers have brought them hot water and food. The rescue operation was ongoing.

A river in Fangshan overflowed its banks Saturday night, trapping more than 40 residents in a village. Twenty have been rescued and rescuers are to reach more.

The rainstorm that started thrashing the city around 10 a.m. Saturday has cut off traffic on some roads inundated by waters and also severely disrupted air traffic.

Authorities have sent out 7,000 traffic police to the city's roads to help cope with traffic problems.

Many roads under overpasses were submerged by waters of up to one meter deep, leaving some cars stranded. Many people therefore chose to take subway rather than drive their own cars.

"That was by no means car driving. It looked very much like sailing a boat," recalled Beijing resident Yuan Xin, who drove back home in rain Saturday.

"There were waves all around," said Yuan. "I was very nervous inside the car and I was wondering now and then what if the car broke down."

Many pedestrians on some roads were seen treading in waist-level waters. While on a road near the Lishuiqiao, many returned after walking into waist-deep water in a 10-square-meter "pond" on the road uncertain how deep the water in front was.

"I read on the microblog that a foreigner swam back home. If the water do not recede soon, I'll swim across it too," a young man holding his leather shoes told Xinhua.

The popular Sina Weibo, a Twitter-like microblogging site, was flooded with photos showing the city being swamped. Many accused that the city's drainage system was ill prepared for rainstorms.

About 475 flights have been canceled and 80 others delayed by more than one hour by 11 p.m., according to the Beijing Capital International Airport. The airport's operation was starting to resume as the rain began to subdue.

The metro line linking the airport with the downtown area resumed operation at 10:05 p.m. after two and a half hours' suspension due to rain-triggered power failure.

The line's service will be extended by one hour to midnight to carry stranded passengers, the metro line's operator said in a statement. Also free shuttle bus service is offered.

According to statistics at 20 state-level climate observing stations in Beijing, the city received 163.7 mm of precipitation on average as of 10 p.m., the largest since weather records began in 1951, said Guo Wenli, director of the climate center under the Beijing Meteorological Bureau. A township in the Fangshan district was hit by the largest rainfall of 366 mm.

The record was to be renewed as the rain continued, Guo said.

The agency issued its first orange rainstorm alert warning since 2005 Saturday evening as the rain is forecast to last over 20 hours till Sunday morning.

Chinese meteorological authorities use a four-tier color-coded weather warning system from "blue", "yellow" to "orange" and "red".

In another orange alert issued at 10 p.m., the bureau forecast that several eastern suburban regions were to receive up to 70 mm of rainfall in the coming three hours.