SHANGHAI  Chinese censors apparently began blocking the news Web sites of CNN, the BBC and the Norwegian broadcaster NRK from appearing in China on Thursday, a day before the Nobel Peace Prize ceremony is to be held in Oslo to honor Liu Xiaobo, the imprisoned dissident.

The Chinese authorities have denounced the decision by the Nobel committee to award this year’s prize to Mr. Liu, who is serving an 11-year prison sentence for subversion after he led a pro-democracy campaign here.

The government has sharply criticized Norway and suggested that Western nations are trying to impose their values on China.

China has not said that it is intentionally blocking the news Web sites of the broadcasters, but Chinese censors regularly black out some portions of CNN and BBC television broadcasts when delicate political topics are covered. This happens even though CNN, BBC and other international television channels are largely limited to five-star hotels and luxury apartments that cater to foreigners.