A veteran democracy activist has become the latest person to be charged with subversion by Chinese authorities in a widening crackdown on dissent that follows anonymous calls for anti-government protests like those in the Middle East and Africa, a rights group said Wednesday.

Though the calls for demonstrations every Sunday have not drawn any overt protesters, the authoritarian government has reacted strongly. Human rights groups say more than 100 bloggers, lawyers, activists and artists have been detained, though only a handful have been formally charged.

Avant-garde artist and outspoken government critic Ai Weiwei is the most high-profile person targeted by authorities since the crackdown began in February.

The Hong Kong-based Information Center for Human Rights and Democracy said Wednesday that the veteran democracy activist Zhu Yufu had become the fourth person to be charged with "inciting to subvert state power" since the anonymous protest calls began circulating. Zhu's wife, Jiang Hangli, was informed of the charges Tuesday, it said in a faxed statement.

Zhu was taken away more than a month ago, on March 5, just days after posting an essay online that declared his support for the protest campaign, the center said.

It said Zhu wrote in his essay that the protest calls could be "the single spark that could start a prairie fire," and bemoaned the worsening of China's social problems.

A man at the Shangcheng District Detention Center in Hangzhou, where Zhu is allegedly being held, declined to comment or give his name, referring queries to the municipal prosecutor's office where an official suggested reporters call the Hangzhou Public Security Bureau. Calls to the public affairs office of the Hangzhou Public Security Bureau rang unanswered.

Zhu, 58, earlier spent seven years in prison for his part in organizing the banned Chinese Democratic Party. He was released in 2006. He was jailed for another two years between 2007 and 2009 on charges of attacking police and interfering in public duties after he and his grown son got in a fight with police.

According to Chinese Human Rights Defenders, another rights group in Hong Kong, the three other dissidents charged with subversion since the crackdown began in February are blogger and writer Ran Yunfei, and democracy activists Ding Mao and Chen Wei. The three are based in southwest China's Sichuan province.

The artist, Ai, was last seen in the custody of officials at a Beijing airport on April 3. His wife, Lu Qing, told the BBC on Wednesday that police still refuse to respond to queries about Ai's whereabouts even though the Foreign Ministry has said publicly he is under investigation for suspected economic crimes.

"We have absolutely no idea what has happened to him or what he's going through right now," she told the BBC. "I am very worried about him."