BEIJING —  The number of people arrested in China on charges of endangering national security more than doubled last year, a rights lobbying group reported Wednesday in a finding that underscores the communist government's sustained clampdown on dissent.

The San Francisco-based Dui Hua Foundation said a recently published Chinese government yearbook showed that state security charges _ commonly used against political critics _ were filed against 604 people in 2006, compared with 296 the year before.

The state security arrests were the highest number since 2002, Dui Hua added.

Only a few of the highest profile prosecutions have been publicized, either by China's government or rights groups, while more than 90 percent of the defendants were not publicly known, Dui Hua said.

The rise in state security charges illustrates the breadth of the government's two-year-old crackdown. The campaign seeks to silence traditional critics as well as new dissent voices emerging from a rapidly changing society and to ensure no disruptions during next summer's Olympics in Beijing.

"This dramatic increase in arrests confirms the heightened crackdown on dissent in China that we've been witnessing since at least the middle of 2005," said John Kamm, a former businessman who founded and runs Dui Hua.

Among the activists known to have been brought up on state security charges are the lawyer Gao Zhisheng, who sought to defend members of the outlawed Falun Gong spiritual group, and two sons of Rebiya Kadeer, a businesswoman now in exile who is an advocate for the rights of her Muslim ethnic group against China's Han Chinese majority.

Also presumably among those charged were 10 Tibetans, another ethnic group that has chafed under the communist government's rule, Dui Hua said.

The crime of endangering state security was enacted a decade ago to replace the more obviously political charge of "counterrevolution." The new charge covers a wide range of crimes from espionage to subversion to advocating separatism.

Dui Hua has compiled an extensive database on Chinese political prisoners. Its researchers often comb through local newspapers and obscure publications to try to identify Chinese sentenced for political crimes, and the organization often lobbies on their behalf.

Based on the recently published 2007 China Law Yearbook, Dui Hua said, 561 of the 604 people charged with endangering state security last year were prosecuted. That was up from 349 in 2005, it said.

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