Chinese police began clamping down on dissent and activists kicked off their annual campaigns Thursday ahead of the 23rd anniversary of the brutal crackdown on the 1989 Tiananmen Square protests.

People's Liberation Army's soldiers stormed into central Beijing on June 3-4, 1989, firing upon unarmed demonstrators and citizens, killing hundreds if not thousands, as they ended six weeks of protests on Tiananmen Square.

Over two decades later, Beijing still considers the incident a "counter revolutionary rebellion" and has refused to acknowledge any wrongdoing or consider compensation for those killed.

Chinese Human Rights Defenders said three people were taken away by police in southwest China Wednesday after organising a meeting to commemorate the Tiananmen massacre.

Yong Zhiming, Mi Chongbiao and Li Kezhen were taken into custody in Guiyang city after the meeting in which they called for the release of veteran activist Chen Xi, who was jailed for 10 years for subversion late last year, said the group.

Although any mention of the protests is banned in state media, calls by the overseas dissident website www.molihua.org Thursday urged those opposed to the crackdown to dress in black and "stroll" in public places throughout China on June 3-4.

The call, which spread through numerous microblogs like Twitter, was like calls last year urging Chinese to hold protests similar to those that spread through the Arab world.

"In the last 20 years or so, fast economic development has created a class of powerful and rich, who have tied up state power through organised violence, robbed the people of their homes and assets and seized the fruits of economic growth from the people," the appeal said.

"There is only one clear response echoing across the expanse of China and through the hearts of the people: end the one-party dictatorship and establish constitutional democracy."

The Tiananmen Mothers, a group of relatives of victims of the 1989 crackdown, issued an annual open letter to the government that called for the end of communist rule and a reassessment of the official verdict on the protests.

"So long as the Tiananmen Mothers exist, our struggle for justice will not cease," the letter, signed by 121 members, said.