Scorching heat has persisted in northern and eastern parts of China since Monday, driving up the number of emergency calls and sending hundreds of people to hospital with heat-related illness.

Beijing, Shanghai and Chongqing as well as cities in Hebei, Henan, Shandong and Jiangxi provinces have issued heat wave warnings, with temperatures soaring to 40 C in many areas on Wednesday.

"The heat wave is expected to move toward the south, with rainfall coming around Friday in the north, cooling down the hot weather," said He Lifu, chief forecaster of the National Meteorological Center.

"Children, the elderly and the infirm are advised to stay indoors due to the soaring temperatures," He said.

Beijing issued this year's first heat wave alert on Tuesday with the temperature soaring to 36 C on Wednesday.

In the capital, ambulance calls surged to about 6,000 times the average every day since Monday.

"Patients with heat-related illnesses surged 23 percent in the first three days of July, compared with early June," said Guan Na from the Beijing Emergency Medical Center.

She suggested people avoid prolonged exposure to the high temperature and to set air-conditioners at a proper temperature, above 25 C.

Cities in central and southern regions, including Hangzhou in Zhejiang province and Chongqing, opened air-conditioned shelters for residents to cool down in community centers.

The soaring temperature is also driving up the sale of fruit, especially watermelons, in Beijing's Xinfadi wholesale market, northern China's largest agricultural produce distribution center.

"The peak season for watermelons is coming," market worker Liu Tong said.

While the dry weather became a good signal for fruit sellers, firefighters in Beijing worry about the dry conditions.

The Shanghai Meteorological Bureau issued a yellow alert on Wednesday — meaning the city's highest temperature would be above 35 C in three consecutive days.

"Wednesday's highest temperature is between 36 and 37 C, slightly lower than Tuesday, but the city won't see any rainfall until Friday and over the weekend," said Fu Yi, a chief service officer with the Shanghai Meteorological Bureau.

According to Fu, the scorching hot days would only be broken by intermittent thunderstorms starting on Friday. On Saturday, the mercury is expected to drop to about 32 C.

Shanghai expects to see about 22 to 27 high-temperature days — which are defined as having temperatures over 35 C — this summer. Only four days with temperatures higher than 35 C have been recorded thus far, including Wednesday.

The mercury hit 38 C on Tuesday in Shanghai, the highest daily temperature recorded so far in Shanghai this year. An orange alert, the second of the three-level color system, was issued that day. Yellow and red are the other color codes, with red being for temperatures that soar beyond 40 C.

Street sweeping has been canceled from 11 am to 2 pm to guarantee the city's 50,000-plus sanitation workers' safety under the sun, said the city's environmental protection authority.

The city's 240 ambulances, which are on day and night shifts, are dispatched more than 1,000 times on an ordinary summer day.

"It's higher at peak times," said Li Minghua, a 35-year-old doctor who works as the trainer for Shanghai Medical Emergency Center. "An ambulance doctor will be sent out at least 12 times during a 12-hour shift.

"The work leaves us totally stressed out, and many develop back or knee injuries from lifting and carrying patients."