One fifth of Bangkok is now under water as Thailand's worst floods in half a century spread through the capital, prompting warnings for affected residents to evacuate, an official said Thursday.

Many people, however, have ignored the advice, choosing to stay in their inundated homes despite risks including electrocution and disease as well as shortages of food and drinking water.

"In terms of area about 20 percent of the capital is under flood water but nobody knows the exact population affected," said a spokesman for the Bangkok Metropolitan Administration, Jate Sopitpongstorn.

"There are 11,000 evacuees living in temporary shelters across the city."

While the centre of the capital remains dry, some northern and western parts have been submerged in dirty water that is waist-deep or higher in places.

Nationwide 437 people have been killed in the disaster, though so far there have been no official reports of deaths in Bangkok.

The authorities have issued an evacuation order for eight of the capital's 50 districts, and for certain areas in four others.

Worst-hit residents have complained that their homes are being sacrificed to save downtown Bangkok's shopping malls, luxury hotels and the houses of the wealthy elite, triggering protests and the destruction of some dykes.

The authorities are attempting to drain the floods through waterways in the east and west of the sprawling metropolis, which is home to 12 million people.

Officials have vowed to do their utmost to protect the centre of Bangkok from inundation, but have been criticised for giving confusing information about the threat level for inner parts of the capital.

The floods -- caused by three months of unusually heavy rains and failure to release enough water from dams in the early part of the monsoon -- have damaged the homes and livelihoods of millions of people.