Wandee stands by the main road, now a knee-deep river, in the Sai Mai district of Bangkok, selling bottles of petrol to drivers as they inch their cars through the muddy water. Like the majority of the capital's residents, she is staying put despite evacuation orders following Thailand's worst floods in half a century and the waters continuing to rise. "We'll stay one more night," she said. "Well, unless it gets really high."

Bangkok faces a growing emergency. Flood defences were breached on Saturday in the Thonburi district, bringing water to the outskirts of the central tourist and business areas. The mighty Chao Phraya river, which wends through the city, is predicted to break its banks over the weekend when coastal tides swell its volume, threatening to inundate central areas.

The government has mobilised more than 50,000 soldiers, 1,000 trucks and 1,000 boats for the relief operation. Army trucks carrying residents through the water pass sandbag barriers stacked like shooting ranges. Along the roadside, entrepreneurs sell everything from wellington boots and lifejackets to blocks of foam and inflatable dinghies. Wandee is proud of the community spirit in the district. "We rely on each other, not the government," she said.

In a number of northern districts of the capital, the Bangkok Metropolitan Authority has ordered evacuations. The city's governor, Sukhumbhand Paribatra, said this weekend was "a critical moment", and that "massive water is coming". Prime minister Yingluck Shinawatra said she was only "50% confident that the inner zone of Bangkok will not be completely flooded".

The government has announced a five-day holiday to enable citizens to escape and emergency relief centres have been set up around the country. With all domestic flights booked up, public transport packed and the roads hectic, it is estimated that hundreds of thousands of people have already left.

There are risks for those who stay. The Red Cross warns that the city faces a "potential humanitarian crisis" of water and mosquito-borne illnesses, and many shops have run out of essentials like bottled water, eggs and instant noodles. Danger also lurks in the water. As well as snakes and rats, there are scores of crocodiles which have escaped from flooded crocodile farms.

Nipapat, a market worker from the north of the city, told how she saw a crocodile attack a small child playing in the water. "The men in the neighbourhood hunted it down and killed it," she said.

The floods have had a devastating impact on the country. More than 380 people have been killed and a third of Thailand's provinces have suffered serious flooding, affecting 2.2 million people. The sheer scale of the problem is laid out in a popular flood-awareness cartoon, which explains that the volume of water, which has to drain from the north to the Gulf of Thailand, is equivalent to the weight of 50m blue whales. The cost of the floods is already estimated at 800bn baht (£16bn) and is likely to rise further.

The city of Chonburi, one hour south of Bangkok, has set up a relief centre in a sports centre to take some of the evacuees from the capital. The gym floor is a colourful patchwork of red mats, dome tents and sprawling bodies. Downstairs hordes of volunteers bustle about, handing out basic supplies, giving financial advice and even organising local trips to lift people's spirits.

For Luangjan, this is the second emergency centre she has been to – the first one was flooded a few days ago. She left Rangsit, north of Bangkok, last week with 10 family members. "The dam broke," she said. "The water was too fast and too strong to save anything." They ferried their family to dry ground in a giant plastic tub, before being picked up by one of the many army trucks criss-crossing the disaster zones.

She is worried about what will happen when they return – her hairdressing business has been flooded and she is sceptical about whether the government will provide any assistance. "When the floods stop they will only care about [big] business – not people like me," she said.

In Bangkok, taxi driver Bprasert was also sceptical about how the government can handle the crisis. He repeated a common criticism that there has been too much political conflict between Bangkok's governor and the prime minister, who both represent different political parties. "One says yes, the other no," he said. "They should work together, but they don't."

Bprasert slept in his taxi on Friday night, parked on the raised express way along with hundreds of other cars left on higher ground by their owners. His district in the south of the city was unaffected yesterday but he was worried that the flood would come soon. He complained that there was not enough information from the Flood Relief Operations Centre (FROC) about where might flood next – and so, unsure of what is going to happen, he prefered to sleep in his car.

With the Chao Phraya river forecast to burst its banks and the prime minister warning that the water in the capital could last a month, the floods provide a real challenge to the new Thai government. For Luangjan, stuck in Chonburi however, it's very simple: "I just really want to go home and tidy up my house."