BEIJING - AT LEAST 20 people have died and more than 670,000 had to be evacuated in China after torrential rain and floods destroyed houses, damaged roads and caused rivers to overflow, state media said on Sunday.

The flooding also blocked roads and left some 300 teenagers stranded at a school with limited supplies of food and water, state media reported.

Flood waters had blocked the entrances to the Hemu Town Middle School in the Guangxi region and rendered nearby roads impassable.

By late Sunday, authorities had managed to restore the power supply and get food and water to the students, an education official, Luo Enwu, told Xinhua.

China Central Television (CCTV) said on Sunday that floodwaters along a major commercial thoroughfare elsewhere in Rongshui were more than 2.5 metres deep. The report said it was the highest water level the county has seen in a decade.

Flood control officials used boats to deliver food, mineral water and other supplies to the school on Saturday, including pumps to lower the water level, according to a Rongshui County official surnamed Lu.

Water levels of some rivers in the southern Guangxi region were the highest since 1998, when downpours led to the worst flooding in over 100 years and resulted in more than 3,600 deaths nationwide, China Central Television said Saturday.

By Friday, 80 per cent of the county was inundated, causing the Rongjiang river to overflow its banks and forcing the relocation of more than 70,000 people, Ms Lu said.

In Guangdong, two construction workers were killed when a rain-soaked wall The county government estimated the damage at 210 million yuan (S$45 million), Xinhua said.

The rain also destroyed a 13-metre section of a dyke near the base of the Kama Reservoir in Guangxi, Xinhua said. -- AP, AFP