Google on Friday reported unexplained disruptions to its service in China.
"We've checked and there is nothing wrong on our end," a Google spokeswoman told AFP.
An online tool that tracks outages or disruptions to the California-based Internet giant's services showed online traffic in China plummet mid-day.
The disruption followed reports that Chinese Internet users have evaded censors to take pot shots at President Hu Jintao's assessment of his performance in a farewell speech.
Hu launched a week-long Communist Party congress in Beijing on Thursday with a speech that touted his political leadership, but also warned in stark terms of worsening problems such as corruption that threaten the party's legitimacy.
Users of the country's hugely popular microblogging sites -- the only major forum for relatively open expression in the tightly-controlled country -- reacted with cynicism.
The GreatFire.org website, which early last year began tracking Internet censorship in China, reported Friday that testing showed an array of Google websites were blocked there including search, email, and maps.
"This means that none of these websites, including Google Search, currently work in China, unless you have a VPN or other circumvention tool," according to a blog on the site, which monitors China's censorship online.
Beijing has denied involvement in the cyberattacks on the California-based firm, calling such accusations "groundless."
Google on Friday confirmed a sharp drop in access to its products in China, noting that it was the middle of the night there and that a clearer picture of the situation should be available as the day got underway.
Chinese censors last month blocked online searches related to the New York Times as well as the newspaper's websites after it published an investigation on the wealth of the Chinese premier's family.
China operates a huge system of Internet control and censorship dubbed the Great Firewall of China, aimed at snuffing out information or comments that the government considers a threat to its authority.
In early 2010, the Google said it suffered cyber-attacks from China-based parties apparently intent on hacking into the Gmail accounts of Chinese rights activists.
The resulting row caused tensions to spike between China and the United States and ended with Google reducing its presence in the Chinese market.