Chinese security forces have boosted their presence in Beijing's Tiananmen Square on the eve of the 20th anniversary of a crackdown on pro-democracy protests.

Chinese plainclothes police and paramilitary forces swarmed around the square Wednesday, examining visitors at checkpoints and barring access to journalists.

China's Communist government sent tanks and troops into Tiananmen Square on June 4, 1989 to crush weeks of student and worker protests. Hundreds and possibly thousands of demonstrators were killed.

Ahead of the anniversary, Chinese authorities forced several leading Chinese dissidents to leave Beijing and confined others to their homes.

Macau protester detained

Authorities in Macau also detained a key student leader of the 1989 protests, Wu'er Kaixi, when he arrived in the southern Chinese territory Wednesday on a flight from Taipei.

In nearby Hong Kong, authorities deported another former student protest leader, Xiang Xiaoji, who flew into the territory from New York.

Xiang lives in exile in the United States and is a U.S. citizen. He had planned to attend an annual June 4 candlelight vigil in Hong Kong to remember those killed in the Tiananmen crackdown. The U.S. consulate in Hong Kong criticized his deportation.

Wu'er Kaixi said he wants to enter mainland China through Macau to see his parents, whom authorities have barred from visiting him in Taiwan, where he lives in exile. He said Macau immigration officials refused to grant him entry and asked him to return to Taiwan.

Web access blocked

China's government also has blocked access to social networking Web sites such as Twitter and foreign media reports to try to prevent discussion of the Tiananmen anniversary.

Rights group Amnesty International said Wednesday China's efforts to cut communication and prevent dissidents from moving will not stop people from marking the anniversary. It says excessive harassment will fuel the quest for truth about the crackdown.

Some information for this report was provided by AFP, AP and Reuters