BlogSpot.com, where people write about their daily lives up to several times a day, has been inaccessible through Chinese networks for a week, they said.
"This is not due to a technical problem,'' said Jason Shellen, business development director at San Francisco-based Pyra Labs, which runs the Web site.
He said bloggers received no explanation, leaving the Internet start-up's staff of six to field queries from frustrated bloggers in China who could update but not read diverse musings ranging from dating to pop music to teenage angst.
The developer of BlogSpot.com could not say how many Chinese Web surfers place or read postings on the site.
The ban comes as a Chinese court and a U.S.-based rights group said an Internet activist in the western region of Xinjiang had been tried for subversion, a crime that carries the death penalty.
Tao Haidong, 45, was arrested in July for posting articles judged objectionable on Web sites. He was tried on Jan. 8 and is awaiting sentencing in the provincial capital of Urumqi, the court and New York-based Human Rights in China said Wednesday.
The rights group said in a statement that state media had accused Tao of slandering Chinese leaders and predicting that China's economy was near collapse.
About 50 million people in China surf the Internet. Authorities openly control Internet and media content in China to protect the Communist Party's unchallenged position, firmly in place since 1949.
Internet policers block several foreign news sites and often force Chinese Web pages to delete content judged objectionable.
An official at the Ministry of Information Industry said the block could be due to material deemed pornographic or detrimental to government interests.
"The Chinese government would never tell Western sites what to post or what not to. They have freedom of expression,'' he said. "So it will just take away access.''
Ben Edelman, a fellow at the Berkman Center for Internet & Society at Harvard Law School, said the Web police barred the site by blocking its Internet-protocol address, analogous to cutting a phone line.
A 20-year-old college student in the southern city of Hangzhou who calls herself "Leylop'' on the Internet said the block would only foment debate and show the Chinese government in a bad light.
"Blockage only causes more dissent," she said. "The bloggers who have something to say won't be deterred by the blockage at all. We'll find other ways.''
Shellen said the start-up was starting to approach Chinese officials to resolve the issue and might seek advice from search engine Google, which suffered a similar block in September.
"We want to proceed with cool heads. We are not so upset that we want to rattle any cages,'' he said.
China blocked access to Google, which has soared in popularity due to its ability to run searches in Chinese, in a crackdown on Web content ahead of a watershed leadership handover in November. The ban was lifted about 10 days later after Google protested.
In March, about 130 major Web portals, including Yahoo,a self-censorship pledge that drew fire from critics who said the sites were sacrificing freedom of expression for the sake of business.
Chinese portals such as Sohu.com have devised a thorough list of terms--including President Jiang Zemin, the Falun Gong spiritual group and Tiananmen Square protests--which are automatically filtered from the site.