While the country's so-called "great firewall" already blocks many sites hosted in other countries, users in Beijing, Shanghai and other parts of China said that they could not reach any foreign sites for about an hour, the British newspaper The Guardian reported.
There were suggestions of a technical explanation, with some saying Wednesday's massive earthquake in the region could have damaged an undersea cable.
"This malfunction is caused by the failure of China's backbone network and is under renovation," an executive at Sohu, one of China's largest Internet portals, wrote in a blog posting.
But others were not convinced.
"Latest news: Most foreign Web sites can't be accessed," the company Data Center for China Internet posted. "Analysis: for commonly known reasons, a large number of foreign URLs are blocked. It is possible that the great firewall is undergoing some readjustment, mistakenly adding many foreign Web sites to the blocking list. The details are unclear."
The fact the problem was cleared up relatively quickly suggested it was not an accident, some said.
"My gut feeling is that it was a software upgrade," said David Wolf of Wolf Group Asia, which advises Internet and other companies.