KARDITSA: Receding floodwaters have revealed a trail of devastation in central Greece left by a rainstorm that raged for three days, killing at least six people and causing billions of euros worth of damage.

Hundreds of people remained trapped on Friday in their homes or on high ground in the central plain of Thessaly, which bore the brunt of Storm Daniel's relentless deluge.

Homes were carried away by torrents, vital infrastructure was destroyed and crops in the country's second-largest tract of farmland wiped out.

"I don't think we have realised the magnitude of this disaster yet," Professor Efthymios Lekkas, a disaster management expert, told state broadcaster ERT on Friday.

The three-day onslaught - in which, meteorologist George Tsatrafyllias said, one region got more rain in 24 hours than London does in an average year - followed a massive wildfire in northern Greece and the country's hottest summer on record.

Prime Minister Kyriakos Mitsotakis was due to visit the crisis-hit area on Friday, his office said. He cancelled a trip scheduled for Saturday to the northern city of Thessaloniki, where he was to deliver his annual keynote speech on the state of the economy.

Speaking to ERT, Thessaly governor Kostas Agorastos said he estimated the storm caused around three times the 700 million euros ($750 million) inflicted by extensive floods in 2020.

More than 1,800 people had been rescued from flood-hit areas across Greece since Tuesday, the fire brigade said.

In the village of Vlochos near the city of Karditsa, about 150 people had been waiting for help for three days.

"We have been on the mountain," Dimitris, one of the stranded villagers, told Skai television. "It's the third day without food, with nothing. We lit fires yesterday to cook whatever we managed to grab from our fridges."

In some areas floodwater were still 2 metres (6-1/2 feet) deep.

One of the breadbaskets of Greece, Thessaly represents about 15 percent of the country's annual agricultural output. It is also a major cotton producing area.

Torrential rains left more than a metre of silt dumped on once-fertile soils. "The agricultural production isn't destroyed just for this year. The thick coat of silt means it is no longer fertile." Lekkas said.