BEIJING (AP) Police detained dozens of activists and petitioners in Beijing and elsewhere in China as President Barack Obama arrived on his first state visit to the country, friends, family members and a human rights group said Monday.

International rights groups have urged Obama to raise human rights concerns during a four-day trip to China that began Sunday night and will include a meeting with President Hu Jintao. China frequently conducts crackdowns on dissent ahead of major events, such as last year's Olympics and this year's National Day celebrations.

Activist Zhao Lianhai, who organized an online support group for parents whose children were sickened by tainted milk last year, was taken away by police from his home late Friday night, his wife, Li Xuemei, told The Associated Press.

Officers also confiscated his computers and other equipment during the raid. Li said they returned the following day with documents for her to sign that said Zhao had been ''criminally detained'' for ''provoking an incident.''

Chen Jianfang, a petitioner from Shanghai who traveled to Beijing with 200 others, said the group wanted to welcome Obama and draw his attention to China's human rights violations. But several dozen of her companions were rounded up by Beijing police when they arrived at the government's petition office Monday, she said.

''They are detaining people everywhere, even if they are only petitioning normally at the state petition office and are not holding any banners or shouting any slogans,'' she said.

Chen said that most of the petitioners have had their homes destroyed and their land taken away without getting fair compensation.

A Hong Kong-based rights group said that more than a dozen activists in Beijing, Shanghai and elsewhere in the country had been detained or placed under house arrest in a bid to muffle their criticism during Obama's visit.

The activists were openly warned against ''making trouble'' during Obama's visit, according to China Human Rights Defenders.

In a statement, the group urged Obama to raise concerns about the practice during his upcoming meetings with Chinese leaders, and to demand the immediate release of those detained in connection with his visit.

''While the government touts its future leaders in letting President Obama meet with a select few students in Shanghai, it is silencing those true leaders who speak out for justice, human rights and the rule of law,'' China Human Rights Defenders said.

Obama held a town hall-style meeting with a group of college students Monday in Shanghai where he emphasized that freedom of expression and other rights were universal values. He flew to Beijing later in the day.