Chinese authorities have rounded up hundreds of activists in the capital Beijing, rights campaigners said on Monday, as they marked the 23rd anniversary of the Tiananmen Square crackdown.

The detentions came as Washington urged Beijing to free all those still jailed over the demonstrations on June 4, 1989, when hundreds, if not thousands, of peaceful protesters were shot and killed by soldiers.

The anniversary of the brutal army action in the heart of Beijing is always hugely sensitive, but particularly so this year ahead of a once-a-decade handover of power marred by fierce in-fighting in the ruling Communist Party.

"They brought in a lot of buses and were rounding up petitioners at the Beijing South rail station on Saturday night," Zhou Jinxia, a petitioner from northeast China's Liaoning province told AFP.

"There were between 600 to 1,000 petitioners from all over China. We were processed, we had to register and then they started sending people back to their home towns."

Police made it clear that the round up of petitioners -- people who gather at central government offices in Beijing to seek redress for rights violations in their localities -- was to prevent them from protesting on June 4, she said.

China still considers the June 4 demonstrations a "counter-revolutionary rebellion" and has refused to acknowledge any wrongdoing or consider compensation for those killed, more than two decades later.

The government attempts to block any public discussion or remembrance of the events by hiding away key dissidents in the run-up to June 4 each year, taking them into custody or placing them under house arrest.

Any mention of the 1989 protests is banned in Chinese state media, and the subject is largely taboo in China. Searches on China's popular social media sites for June 4, the number 23 and the word "candle" were blocked on Monday.

Despite the heightened security, numerous public events have been held around the nation to commemorate the "Tiananmen massacre" and demand democratic reforms.

More than 80 rights campaigners met in a Beijing square on Saturday, carrying banners and shouting slogans calling for a reassessment of the 1989 protests.

"We shouted 'down with corruption', and 'protect our rights'," Wang Yongfeng, a Shanghai activist, who attended the protest, told AFP.

"So many people were killed on June 4, we think the government should fully account for what happened."

Photographs of the Saturday protest posted online showed demonstrators with large placards that said "remember our struggle for democracy, freedom and rights as well as those heroes who met tragedy."

A similar protest occurred in a park in southeast China's Guiyang city last week, with police subsequently taking into custody at least four of the organisers of the event, the Chinese Human Rights Defenders group said on its website.

In Beijing, veteran dissident Hu Jia said on his microblog that, as in previous years on the Tiananmen anniversary, police had stepped up security around the homes of numerous political activists and social critics.

Rights activists and lawyers said police had also contacted them and warned against participating in activities marking the crackdown.

Another rights defender, Yu Xiaomei from eastern Jiangsu province, told AFP by telephone she had been followed by three men when she left her home on Monday.

"I recognised one of them. He had beaten me and detained me two years ago. I ran away, I don't dare go out onto the street today," she said.

The only open commemoration of the crackdown to be allowed on Chinese soil will take place in Hong Kong, a Chinese territory that enjoys freedoms not allowed in the mainland.

Organisers say they expect more than 150,000 people to join a candlelight vigil to mark the anniversary.

burs-cc/mtp