Google suffered intensive disruption in China tonight just days after it was warned by the authorities to scale back its search operations.
Search functions and Gmail were inaccessible for more than an hour in a move seen by web watchers as a warning shot across the bows by China's censors.
"This is definitely a warning to Google, as well as other foreign companies," said Xiao Qiang, the founder of China Digital Times. "It is also a strong warning to Chinese netizens. The government is showing its determination to keep the internet under control."
Earlier in the day, the main state and communist party media - Xinhua and People's Daily - condemned Google for providing links to pornographic websites through its search engine. Last week, the government ordered the US company to halt foreign website searches as a punishment.
Many Chinese netizens believe the move is intended to distract attention away from the domestic controversy over Green Dam censorship software, which must be sold with all new computers from 1 July.
In a rare move, the US has lodged a complaint over the tightening of censorship rules. Google agreed to self-censor in compliance with requests by local officials after setting up a China subsidiary and locally hosted website in 2005. One reason for this controversial decision was that its services were frequently being disrupted or slowed. That has been rare since.
The extent of the latest blockage was not immediately apparent, but the head of Google's China operations, Kai-Fu Lee posted a call for patience online: "We have received your feedback, we are just investigating, please be patient and wait for feedback from Google overseas, thanks for your support."
Frustration against censorship is high. Angry Chinese bloggers have called for a boycott of the internet on 1 July.