Millions of nervous Bangkok residents were warned Friday to move their belongings to safety as the kingdom's worst floods in decades began pouring into the northern outskirts of the sprawling city.

In a desperate attempt to drain the mass of muddy water, the authorities have opened all of Bangkok's sluice gates to allow the floods to flow through canals and rivers in the low-lying capital and into the Gulf of Thailand.

The move should ease pressure on vulnerable flood barriers on the northern edge of the city of 12 million people, but increases the threat to Bangkok itself, where some outlying residential areas were inundated on Friday.

People were advised to move their possessions to higher floors or safe areas after the government admitted the sea of water bearing down on the capital from the central plains was unstoppable.

"I ask all Bangkok residents to move your belongings to higher ground as a precaution, but they should not panic. It's preparation," said Prime Minister Yingluck Shinawatra, who has described the crisis as "overwhelming."

Three months of heavy monsoon rains have killed at least 342 people in Thailand and damaged the homes and livelihoods of millions of others, mostly in the north and centre.

Tens of thousands of people have been forced to seek refuge in shelters, including 33-year-old Nonglak Yodnankham who fled the approaching water in Pathumthani province just north of Bangkok.

"The flood was following behind us when we ran away. Within five minutes, it was already up to my waist," she told AFP at an emergency shelter at Bangkok's number two airport, Don Mueang.

The waters were already seeping into the capital too, leaving little doubt that large areas of the metropolis would be inundated. The only uncertainty was how deep the floods would be and whether the city centre would be hit.

Water up to 70 centimetres (28 inches) deep submerged roads and houses elsewhere in Don Mueang district while more than 2,000 homes were flooded in neighbouring Lak Si, about 15 kilometres (nine miles) from the city centre.

"The water level keeps rising," Don Mueang district chief Phumpat Damrongkiatisak said, adding that the floods already covered several square kilometres (miles) in his area alone.

The authorities reassured residents they had evacuation plans ready if necessary.

Yingluck said the authorities would organise additional evacuation centres, make more parking spaces available and assign security officials to oversee significant locations such as the palace and Bangkok's main airport.

Bangkok residents have rushed to stock up on food and bottled water, while motorists have parked hundreds of cars on bridges or elevated roads.

Tens of thousands of soldiers and police have been mobilised to maintain order.

The opposition is calling on the government to declare a state of emergency to make it easier to control people and prevent them damaging dykes to ease the flooding in their own areas -- but Yingluck ruled out such a move.

Amid signs of tensions between the government and the Bangkok governor, the 44-year-old former businesswoman said she would invoke a section of the disaster law to increase her authority in dealing with crisis.

Yingluck, who is the sister of fugitive former leader Thaksin Shinawatra and was a political novice before taking office, is facing the first major crisis of her two-month-old leadership and has shown signs of strain.

The authorities have failed to protect a number of major industrial parks from the gushing brown water, which has inundated hundreds of factories outside Bangkok, disrupting production of cars, electronics and other goods.

The government says more than half a million people have seen their jobs disappear for now.

Most of Thailand's main tourist attractions -- including the southern islands of Samui, Phuket and Phi Phi -- have been unaffected although some foreign governments have warned against non-essential travel to Bangkok.

Bangkok's main airport, built on a drained marsh, is still operating as normal and its flood defences have been reinforced.