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.