At least 51 people have been killed in flash floods sweeping the eastern Spanish region of Valencia following torrential rain on Tuesday that left roads and towns under water, local authorities said on Wednesday.

Rescuers using dinghies worked in the dark to scour the floodwaters, rescuing several people, television pictures from the town of Utiel showed, and emergency services were still working to reach the worst-hit areas.

Carlos Mazon, the regional leader of Valencia, said some people remained isolated in inaccessible locations.

Dozens of videos shared on social media overnight appeared to show people trapped by the floodwaters, with some climbing into trees to avoid being swept away.

Emergency services in the region urged citizens to avoid all road travel and to follow further updates from official sources, and the a military unit specialised in rescue operations was deployed in some places to help local emergency workers.

Spain's state weather agency AEMET on Tuesday declared a red alert in Valencia, a major citrus-growing region, with some areas such as Turis and Utiel recording 200 mm (7.9 inches) of rainfall. It has since lowered the alert to yellow as the rain had largely stopped.