Liu Xiaobo was arrested Tuesday on suspicion of "inciting to subvert state power," which carries a maximum penalty of 15 years in jail. It was the highest-profile arrest of a Chinese dissident since human rights activist Hu Jia was detained last year ahead of the Beijing Olympics.

On Thursday, about 30 activists led by several Hong Kong opposition lawmakers chanted "Free Liu Xiaobo immediately" and "We oppose criminal convictions based on speech" outside the Chinese government's liaison office in the territory. Hong Kong is a Chinese-ruled former British colony that enjoys freedom of speech under a semiautonomous political system.

"We are worried if Liu Xiaobo will get a fair trial and get enough legal help," the protesters said in a statement.

Liu was among more than 300 lawyers, writers, scholars and artists who signed "Charter 08" in December calling for a new constitution guaranteeing human rights, election of public officials, freedom of religion and expression, and an end to the Communist Party's hold over the military, courts and government.

It also called for the abolition of the criminal code that allows people to be imprisoned for "incitement to subvert state power" _ the charge Liu now faces.

Liu was detained a day before the manifesto was published.

In his writings, most published only online, the 53-year-old former professor has called for civil rights and political reform, making him subject to routine harassment by authorities.

He also previously spent 20 months in jail for joining the 1989 student-led protests in Tiananmen Square, which the Chinese military crushed, killing at least hundreds.

In a statement Thursday, former Tiananmen Square student leader Wang Dan, who lives in exile in the U.S., said Liu's arrest showed China remained a hard-line regime despite two decades of capitalist-style economic reforms.

"Even though Chinese society is changing, even though the Chinese economy is growing, the fundamental nature of the Chinese Communist Party's authoritarianism and their use of state violence to maintain their way of governance hasn't changed," Wang said in the statement.

Democrat Nancy Pelosi, the leader of the U.S. House of Representatives, said in a statement Wednesday that Liu's arrest "deserves the full condemnation from the international community."

"The Chinese government should cease its efforts to suppress the Charter 08 movement and listen to the many Chinese people calling for political reforms," Pelosi said.