“The intention of his writing is clear — he wanted to educate people about democracy and universal values, and has influenced many young people,” said Weican Meng, a friend of Mr. Yang’s and the founder of Boxun News, a Chinese-language website in the United States.

“Before he went back to China, we had a meal together and a number of friends told him it’s not a good time to go,” added Mr. Meng, whose pen name is Wei Shi. “The situation in China right now is a bit like during the Cultural Revolution: People are being punished for talking about very minor things.”

On Thursday, China’s minister of public security, Zhao Kezhi, told a meeting of police commanders in Beijing to guard against political subversion and attempts to foment “color revolution” against the government.

Mr. Yang’s family and friends believe Mr. Yang is being held in Beijing.

Feng Chongyi, a friend of the writer’s and an associate professor at the University of Technology Sydney who was himself detained by the Chinese authorities in 2017, said he had spoken to Mr. Yang’s relatives. They told him that Mr. Yang had landed in Guangzhou early Friday morning, but that he did not make his planned connection to Shanghai, Mr. Feng said.

According to Mr. Feng, Mr. Yang went to China in part because his United States visa was to expire in a few months and he was waiting for a residence visa in Australia for his wife and stepdaughter. They had traveled with him to China on this trip.

According to Mr. Feng’s conversations with the writer’s relatives, Mr. Yang and his wife were interrogated for over 12 hours — probably at the airport in Guangzhou — before Mr. Yang’s wife was then allowed to go to Shanghai to drop off her daughter.

“At home in Shanghai,” Mr. Feng added, “she was in tears and asked relatives to not contact them again, but said she would post their whereabouts.”