The extreme weather system has destroyed more than 15,000 homes, sending 78,000 people to state-run temporary shelters, the Disaster Management Centre (DMC) said.

President Anura Kumara Dissanayake invoked emergency laws, granting him sweeping powers to deal with the devastation after a week of torrential rain across the island.

"We have 132 confirmed dead and another 176 missing," DMC Director-General Sampath Kotuwegoda said, adding that relief efforts had been bolstered with the deployment of the army, navy and air force.

The military rescued 69 bus passengers on Saturday, including a German tourist, who were marooned in the Anuradhapura district after a 24-hour operation involving a helicopter and naval boats.

One of the passengers, speaking to reporters at a local hospital, said navy sailors had helped them climb onto the roof of a nearby home after using ropes to help them safely wade through flood waters.

"We were very lucky... while we were on the roof, a part of it collapsed... three women fell into the water, but they were helped back onto the roof," Shantha said.

A helicopter had to abort an initial rescue attempt as the downdraft from the rotors threatened to blow away the roof they were perched on, he said.

They were later rescued by naval boats.

Roads in the central district of Badulla remained inaccessible, leaving many villages cut off and relief supplies unable to get through.

"We lost two people in our village... others are sheltering at a temple and a house that is still standing," said Saman Kumara from the village of Maspanna in Badulla, one of the worst-affected districts.

- No clean water-

"We can't leave the village and no one can come in because all roads are blocked by earth slips. There is no food and we are short of clean water," he told media website News Center by telephone.

Officials said about a third of the country was without electricity and running water as power lines had collapsed and water purification facilities were inundated. Internet connections were also disrupted in many areas.

Cyclone Ditwah moved away from the island on Saturday and was heading towards neighbouring India to the north.

India's Chennai Airport has cancelled 54 flights in view of the cyclone's approach, with the weather department forecasting extremely heavy rainfall and strong winds over the next 48 hours.

Fresh landslides hit the central district of Kandy, 115 kilometres (71 miles) east of Colombo, with the main access road underwater at several locations.

The government has issued an appeal for international help and asked Sri Lankans abroad to make cash donations to support affected communities.

Officials said Prime Minister Harini Amarasuriya had met Colombo-based diplomats to update them on the situation and seek assistance from their governments.

India was the first to respond, sending two plane loads of relief supplies, while an Indian warship already in Colombo on a previously planned goodwill visit donated its rations to help victims.

Indian Prime Minister Narendra Modi expressed his condolences over the loss of lives in Sri Lanka and said New Delhi was ready to send more aid.

Flooding in low-lying areas worsened on Saturday, prompting authorities to issue evacuation orders for those living along the banks of the Kelani river, which flows into the Indian Ocean from Colombo.

Rain had eased in most parts of the country, including the capital, but the island's north was still experiencing showers due to the residual effects of Cyclone Ditwah.

The cyclone is Sri Lanka's deadliest natural disaster since 2017, when flooding and landslides killed more than 200 people and displaced hundreds of thousands of others.

The worst flooding Sri Lanka has experienced since the turn of the century occurred in June 2003, when 254 people were killed.