The death toll from the worst rains to hit Beijing in more than 60 years has risen to 77, China's official Xinhua news agency said Thursday, more than doubling previous figures.

Many residents of China's sprawling capital had expressed doubts about the official toll of 37 that Beijing authorities announced late Sunday, believing the true figure to be much higher.

Saturday's freak downpour, said to be the heaviest rain since records began in 1951, caused rivers to burst their banks and flood major highways, submerging large numbers of vehicles.

In the worst-hit area of Fangshan, on the city's mountainous southwestern outskirts, distraught residents on Monday reported cars being swept away and said many people were still missing.

Since then, accounts have emerged of drivers stuck inside their vehicles, unable to open doors and windows as water levels surged, with rescue workers slow to reach them.

Many people took to China's weibos -- Twitter-like microblogs -- to condemn the official response to the disaster in the capital, which came at a time of heightened political sensitivity ahead of a 10-yearly handover of power.

Some said the number of deaths and scale of destruction could have been lessened if the government had issued better warnings, including by SMS, and modernised Beijing's ancient drainage systems.

Authorities ordered state media to stick to stories "worthy of praise and tears", while censoring the nation's voracious microblogs and threatening arrests.

"From today onward, we will severely strike at those using the Internet to... create and transmit political rumours and attack the (Communist) party, state leaders and the current system," the Beijing Times quoted city police chief Fu Zhenghua as saying Tuesday.

The threat, reported widely Thursday, did not appear to stifle critical comments, with one typical posting calling it "an open confrontation with the people".

Beijing city spokeswoman Wang Hui insisted earlier this week that authorities would not cover up the true number of deaths, acknowledging that the lack of official updates had given rise to public suspicion.

She said authorities recognised the importance of disclosing casualty figures, citing the SARS (Severe Acute Respiratory Syndrome) epidemic of 2003, when China faced an international backlash for trying to cover up the disease.

The floods caused over 10 billion yuan ($1.57 billion)-worth of damage and affected more than 1.9 million people, Xinhua also said.

City authorities said this week that Beijing's mayor Guo Jinlong would step down, after many weibo users called for his resignation.

Authorities however said the move was unrelated to the floods and that Guo was moving to become Beijing's Communist party secretary, a more senior role.