A Japanese man who returned home after a six-year imprisonment for espionage in China says the country's authorities will likely step up crackdowns on protests over the government's anti-coronavirus measures.

Suzuki Hideji, who belongs to an organization that promotes ties between Japan and China, made the remark during a news conference in Tokyo on Wednesday.

Suzuki was detained in Beijing in 2016 on suspicion of endangering the nation's security. A Chinese court later ruled that he was involved in spying activities, and handed down a prison sentence of six years.

He returned to Japan last month after serving his term.

Suzuki reportedly had visited China about 200 times before he was detained, and taught at a university there.

He criticized the ruling, saying his hearings were held behind closed doors, and that requests to call witnesses were all rejected.

Suzuki mentioned the protests against the Chinese government's "zero-COVID" policy that have erupted in cities including Beijing and Shanghai.

He said China is very wary of mass movements after the Cultural Revolution and the Tiananmen Square incident.

Suzuki said the authorities will likely intensify crackdowns as some protesters are calling for President Xi Jinping to step down.

He also said the Chinese Communist Party is wary because people are starting to demand various freedoms, such as those of expression and thought, as they become wealthier and their sense of values changes.