Google has blamed the Chinese government for problems accessing its e-mail service in the country.
Internet users have complained that the authorities have stepped up disruption of its Gmail service in recent weeks.
Google said it had found no technical issues, and blamed "a government blockage carefully designed to look like the problem is with Gmail".
Users say the interference coincided with an internet campaign calling for protests like those in the Middle East.
Last year, Google said it suffered cyber-attacks from China-based organisations intent on hacking into the Gmail accounts of Chinese rights activists.
The incident caused tensions between China and the United States, and led to Google reducing its presence in the Chinese market.
Beijing has always denied any state involvement in the cyber-attacks, and has in the past called such accusations "groundless".
But problems resurfaced this month with Google writing in its official blog about hackers exploiting a vulnerability in Internet Explorer to attack some Gmail users.
"We've noticed some highly targeted and apparently politically motivated attacks against our users. We believe activists may have been a specific target," it wrote.