The deluge of rain that hit Wales over the weekend continued to wreak havoc on Sunday night, with hundreds of people evacuated from their homes, while others returned to their houses to begin the long process of clearing up.

Six hundred people were temporarily evacuated from the village of Pennal, in Gwynedd, amid fears that a reservoir dam was at risk of collapsing following a landslide.

There were reports that a small breach in the dam half a mile upriver had already allowed some water to escape. Police said that a controlled release of water had been undertaken and was continuing to reduce the water level, so it was hoped that people would be able to return to their homes during the evening.

The temporary evacuation came after a weekend of heavy rain caused havoc across much of west Wales.

About 150 people were evacuated as up to 1.5 metres of water swamped the area around Aberystwyth on Saturday. More than 13cm (5ins) of rain, twice the local average for June, fell in 24 hours and floodwater was still in evidence on Sunday.

Andy Francis, of Mid and West Wales fire and rescue service, said there was "mass-scale damage to caravan parks and private dwellings" across the area. "A lot of floodwater has gone through them, leaving a huge amount of damage, and a residual danger as well from the biohazards, from sewerage and other contaminants.

"Lots of sewers may have been damaged, and gas and water supplies damaged, so my advice to anybody entering their properties this morning is to take sensible precautions."

Francis said high river levels remained a risk. "Please do not go near the water, it's still extremely dangerous, and don't try to drive through it either, because you will end up becoming a casualty and requiring rescue."

Caravans and tents at Aberystwyth holiday park remained unreachable on Sunday, and holidaymakers were forced to assess the damage from a distance.

Simon Saunders, a tourist from the Midlands, was staying in a caravan with friends when they were told to evacuate the park on Saturday. "It's amazing no one was seriously hurt when you consider the damage done. The caravan parks are popular with families and retired people so were the evacuation not done so quickly someone could have easily got hurt. My mate's camper van looks as if it's stranded and we won't be going home today."

Alun Williams, 64, said there was a general feeling of shock. "Only a couple of weeks ago we were in the news as the Olympic torch came through Aberystwyth. Now the town's in all the papers because of the floods. Yes, it could have been a lot worse, but I've not heard one person say how lucky they are. I just hope it doesn't put people off from coming back."

About 25 people from Pennal were being looked after at the Machynlleth leisure centre, said the assistant manager, Aled Davies.

He added: "The people are in good spirits. We are looking after them, they are having refreshments and watching TV. We are not sure how long they are going to stay here – we are taking each half hour as it comes. They will be able to stay all night if they need to. We have temporary beds. We have four or five people looking after them, local staff are helping, and we have offers of assistance from the local community."

The first minister of Wales, Carwyn Jones, praised the emergency crews. He said: "Having spent a number of years living and studying in Aberystwyth, I was deeply shocked to see the extent of the damage caused by the flooding in an area I know so well.

"Yet again we have witnessed the impressive bravery of our emergency services. Working in truly harrowing conditions they ensured up to 1,000 people were moved to safety. It is thanks to the professionalism and dedication of both our professional and volunteer emergency crews that no lives have been lost."

A number of rivers burst their banks as the stormwater flowed downstream. Rescuers helped people to safety at the Holiday Park and Riverside Caravan Park in Llandre, Mill House Caravan Park at Dol-y-bont, Sea Rivers caravan park, Ynyslas, and Glanlerry Caravan Park, near Borth.

Flash floods also affected two villages near York, flooding roads and homes. North Yorkshire fire crews said they had pumped water out of Flaxton and Stockton-on-the-Forest after torrential rain on Saturday afternoon.

In Durham the second oldest regatta in the country was cancelled on Sunday because of dangerously high water levels on the river Wear. One flood warning remained in place in Wales on Sunday – for the river Teifi at Lampeter and Llanybydder.

The Environment Agency advised people of the possibility of more flooding across parts of northern England on Sunday, and in southern England overnight and into Monday. Because of the complex weather, though, there was still some uncertainty as to which areas were most at risk.