Heavy showers upstream inundated the banks of the Kelani on Friday night, and the situation deteriorated rapidly the next day even though Cyclone Ditwah -- which brought the rains -- had already moved on.

Most residents along the banks of the major waterway in Kolonnawa ignored repeated flood warnings, thinking it wouldn't be as bad as authorities were predicting.

Climate change has increased the intensity of storms, and produced more heavy rain events because a warmer atmosphere holds more moisture.

Delivery driver Dinusha Sanjaya said he brushed aside the warnings, assuming the worst would be a few feet of water.

But within an hour, his two-storey house was submerged.

"I never thought the floods would be this bad," Sanjaya, 37, told AFP at Vidyawardana school where all his neighbours were also taking refuge on Monday.

"Every year we experience minor floods, but this is something else. It is not just the amount of water, but how quickly everything went under."

Neighbour Fatima Rushna, 48, said she realised the house was flooding when water reached her bed shortly after midnight on Saturday. She rushed out with her husband, Mohamed Azmi, 50.

"We had no time to collect any valuables. All we have are the clothes we are wearing," she said while waiting for donations at the makeshift camp.

- Livelihoods lost -

For C. V. Ariyaratne, 70, and his wife Emalin, 65, evacuating in a hurry was particularly challenging as she suffers from scoliosis, is severely hunched and needs help to walk.

"We have been through floods, but this is even worse than what we experienced in 2016," Ariyaratne said, referring to the disaster nine years ago when 71 people were killed across the country.

For seamstress Nirushika, 44, the floods have taken away her livelihood.

"I earned a living by running a small sewing business," she said. "Both my sewing machines were lost."

There was more havoc upstream.

The mountainous central region was the worst-affected, with scores of people buried alive in mudslides triggered by record rain, in some places over 500 millimetres.

- 'Clean-up' -

Official data showed that 257 out of the 355 deaths so far were in the central hilly tea-growing region. Another 366 were still missing.

President Anura Kumara Dissanayake has declared a state of emergency to deal with the disaster and vowed to "build back better" with international assistance.

Residents at the Kolonnawa camp said the state had provided dry rations, which volunteers had cooked for them.

Classrooms in the three-storey building had been turned into makeshift accommodation, occupied by about 300 people, including 80 children below the age of 12.

As the situation elsewhere in the capital slowly returned to normal, residents were seen donating food and other essentials.

Government figures show that nearly 210,000 people were in camps, while another 1.15 million needed some form of government assistance.

The Irrigation Department said water levels in Colombo had peaked and that the floods should begin receding within a day.

"Colombo floods are at their maximum now," Irrigation Director L. S. Sooriyabandara said. "Flood levels elsewhere have gone down substantially."

Survivor G. Patrick, 60, said he was dreading returning home.

"My mother, who is 80, is blind and I am not able to clean the house on my own," Patrick told AFP.

"For me, the biggest problem is getting help for the clean-up."