Chinese authorities have been on high alert for large public gatherings in the wake of anonymous online calls for anti-government protests modeled on demonstrations in the Middle East and North Africa.
No major protests have occurred in China following the calls, but the security crackdown they sparked has resulted in the arrest or detention of dozens of public interest lawyers, writers, intellectuals and activists.
Ai Weiwei, an internationally known avant-garde artist who is also an outspoken government critic, became the highest-profile person targeted in the sweep when he was apparently detained at a Beijing airport a week ago. The Foreign Ministry says he is being investigated for alleged economic crimes, though Beijing police have yet to confirm he is in custody.
Ai was last seen being led away by police at the airport after being barred from boarding a flight to Hong Kong.
About 50 pro-democracy protesters in Hong Kong on Sunday demanded Ai's release, peacefully chanting "No to political persecution" outside the central Chinese government's liaison office. Opposition legislator Lee Cheuk-yan tossed a picture of Ai into the grounds of the compound.
Former British colony Hong Kong enjoys Western-style civil liberties as part of its special semiautonomous status under Chinese rule.
On Friday, U.S. Secretary of State Hillary Rodham Clinton called for Ai's release and criticized China for what she said was a deteriorating human rights situation in the first part of 2011.
Clinton made the remarks while announcing the release of the U.S. State Department's annual assessment of human rights around the world. It said China stepped up restrictions on critics and tightened control of civil society in 2010 by limiting freedom of speech and Internet access.
China blasted back at Washington on Saturday with a statement on the Foreign Ministry's website saying the U.S. should reflect more on its own domestic rights abuses.
"The U.S. should stop interfering in other country's internal affairs with this human rights report," ministry spokesman Hong Lei was quoted as saying.
Associated Press videographer David Wivell in Beijing and AP writer Min Lee in Hong Kong contributed to this report.