In a rare June heat wave, 12 provinces saw temperatures soar above 35 C on Wednesday while high temperature alerts were issued by the national weather forecaster.

The unusually warm weather was expected to continue in southern China through the week, but northern temperatures should decrease with rainfall expected on Friday, said He Lifu, chief forecaster at the National Meteorological Center.

"The abnormally high temperatures are largely due to subtropical high pressures, which typically bring summertime heat to the middle and downstream regions of the Yangtze River," He said.

Temperatures reaching 37 C lingered in places including Shaanxi, Chongqing, Hubei, Hunan, Jiangxi, Zhejiang and Guangdong on Wednesday, according to the weather center.

Chongqing had its hottest days of the year on Monday and Tuesday as temperatures passed 41 C, according to the local weather center.

The Chongqing education commission issued a notice on Monday to county and district level education bureaus allowing schools to close because of the heat - with government approval - and heavy outdoor activity was forbidden until temperatures fell.

"It's the first time the school closed in June because of hot weather. This usually happens in September," said Liu Gang, a 29-year-old primary school teacher in Chongqing.

But not all Chongqing students had time off because of heat. Some senior high schools held classes as usual.

"The classroom is as hot as a sauna!" complained Deng Yunxin, a sophomore at Chongshengqiao senior high school.

In Hunan province, the first orange heat alert of the year was issued on Wednesday morning, warning that temperatures were expected to pass 38 C before Thursday. An orange heat alert ranks the second highest on the country's heat warning chart.

In Wuhan, one of China's hottest cities, the civil affairs department is expected to invest 7.3 million yuan ($1.2 million) on running the city's 1,460 indoor shelters to help residents stay cool, Zhang Mingwu, deputy director of the department, said in People's Daily on Wednesday.

Every shelter will have at least one air conditioner, cold drinks, medicines for heat stroke, entertainment, sterilization facilities and firefighting devices, according to the bureau.

The free shelters will be opened whenever temperatures reach 35 C and above.

High temperatures drive up demand for electricity in many provinces.

Power demands in Hunan reached 300,000 megawatt-hours as homes and businesses turned up their air conditioners on Monday. The average daily demand peak is expected to reach 420,000 megawatt-hours this summer, according to Hunan Electric Power Corp.

Xue Zhenyu in Wuhan contributed to this story.