Most of the nucleic acid testing centers in Beijing have been set up outdoors in stadiums, exhibition centers or open spaces near residential areas. Some of the centers operate until midnight to ensure the tests are completed as quickly as possible.

Cen Yunpeng, 26, had a throat swab taken in a residential area in Fengtai on Tuesday. He had gone to Xinfadi market on June 3 to buy salmon for a family gathering to celebrate his birthday.

After the government urged people who had been to the market to come forward, he immediately contacted community workers. He was instructed to stay at home and wait for a nucleic acid test.

"I was told to go to the designated testing spot on Tuesday afternoon. The whole process was very organized. I can learn the result by scanning the QR code they gave me," said Cen, who is still waiting for his results.

Beijing has been sweltering under a heat wave in recent days, which has made collecting samples sweaty and tiring work for medical workers who are required to wear personal protective equipment. The city issued a heat wave yellow warning for Sunday to Tuesday. Some medics had cases of heatstroke as the temperature hovered just under 40 C. "I only waited for about 15 minutes and I was sweating like a pig because of the heat. For the medics it must be like a steam room in those protective suits," Cen said.