The June 30 detentions appeared to mark the largest roundup of Japanese citizens engaged in suspected illegal activity in China in years.
Foreign ministry spokesman Geng Shuang said today that the Japanese government had been informed of the detentions, but gave no details.
China has been cracking down on telecoms scams targeting Chinese victims, many of them run from outside the country. Scammers send false bank notifications or other messages demanding that recipients transfer money to separate accounts to avoid penalties or win rewards.
Japan's Kyodo News agency said the scammers phoned victims in Japan. It said the 35 were detained in the southeastern province of Fujian earlier this month and Japanese authorities alerted on July 3.