Dozens of people remain imprisoned for taking part in the 1989 pro-democracy protests centered in Tiananmen Square, though releasing them would improve China's image ahead of the Beijing Olympics this summer, a human rights group said.

In Hong Kong, tens of thousands of activists gathered at Victoria Park with white candles to mourn those killed at Tiananmen, chanting slogans calling for democracy and the release of political dissidents.

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"Despite being awarded the Olympics, the Chinese communist government has by far not improved its human rights record," Lee Chuek-Yan, a lawmaker and pro-democracy activist, told demonstrators, estimated by organizers at around 48,000.

Discussion of the student movement and the June 3 to June 4 military assault on the protesters in which hundreds, possibly thousands, were killed remains taboo within China. The Communist leadership labeled the protest an anti-government riot and has never offered a full accounting of the crackdown.

New York-based Human Rights Watch also has urged China to free Tiananmen prisoners. "The Chinese government should show the global Olympic audience it's serious about human rights by releasing the Tiananmen detainees," Sophie Richardson, the group's Asia advocacy director, said in a statement released Monday in New York.

Human Rights Watch said about 130 prisoners are still being held for their role in the demonstrations that were crushed in a brutal military crackdown. The square in the heart of the Chinese capital is expected to feature prominently in media coverage of the Olympics, although authorities worry about the possibility of fresh protests marring the event.

The square was calm Wednesday morning on the 19th anniversary of the June 3-4 military assault on the protesters in which hundreds, possibly thousands, were killed. China's Communist leaders portray the protest as an anti-government riot and have never offered a full accounting of the crackdown.

In a slight increase in the normally tight security, uniformed police and other security officials patrolled the square. There were random bag checks and plainclothes police used handheld video cameras to monitor the scene.

The only visual reminder of the protests 19 years ago was that the Monument to the People's Heroes was roped off and guarded. The monument was used as a rallying point by the students in 1989.

In Washington, the U.S. State Department urged China to make a full public accounting of those killed, detained or missing in the crackdown. It called on the international community to urge China to release prisoners still serving sentences from the protests.

The U.S. said that Chinese steps to protect freedoms of its citizens would help it "achieve its goal of projecting a positive image to the world."

China pledged to improve its human rights situation in its bid to host the 2008 Olympics. But one Tiananmen activist, whose son was killed as he hid from soldiers enforcing martial law, scoffed when asked whether the August games had spurred the government to change its attitude.

"I don't have this kind of illusion," said Ding Zilin, pointing out that some of her activist friends were placed under house arrest this year. She is the co-founder of the Tiananmen Mothers, a group representing families of those who died, and has campaigned to get the government to acknowledge those killed in the crackdown and compensate their families.

No official figure is available of the number of people who remain jailed in connection with the protests. Liu Xiaobo, an Internet writer who was jailed for nearly five years after the protests, said he knows of up to eight people serving life sentences in Beijing's No. 2 Prison on charges of organizing people to oppose the soldiers or taking part in acts such as the burning of police or army vehicles during the crackdown.

China Human Rights Defenders, a network of activists and rights monitoring groups, released a list Tuesday with the names of eight Beijing residents who remain imprisoned in connection with the Tiananmen protests. A handful of activists had also been placed under house arrest or monitored by police in the days leading up to the anniversary, the group said.

Human Rights Watch said its numbers represented its best estimate based on government information and details pieced together from other sources like family members and other activist groups.

Foreign Ministry spokesman Qin Gang declined to comment on prisoner numbers or any possible future releases, calling that a matter for other government departments.

He repeated China's position that the government has been improving human rights by reforming the legal system and raising living standards.

"We will unswervingly stand on the road of socialism with Chinese characteristics and the promotion of human rights of China," Qin said at a regularly scheduled news conference.

Security is traditionally ratcheted up around the anniversary, but Liu said this year has been relatively relaxed, with police only appearing outside his Beijing apartment on June 1, several days later than usual.

Liu is still unable to publish inside China, his phone is bugged and he is frequently called in by police for discussions on topics ranging from unrest in Tibet to the government response to last month's earthquake in the southwestern province of Sichuan.

Despite a public taboo on discussing the protests and crackdown, Liu said interest in the events appears greater now than on the 10th anniversary in 1999.