Twenty years after pro-democracy protesters met the might of China’s military in Tiananmen Square, a massive security operation has been put in place to prevent any reenactment or celebration of the event.

Several hundred police, paramilitary and other security personnel swarmed over the square in the heart of Beijing, where the army crushed weeks of protests on the night of June 3 and 4 in 1989.

The anniversary of the army’s Tiananmen crackdown – in which hundreds, possibly thousands of people were killed – is sensitive for the central government, which has made any discussion of the events of June 1989 taboo.

Several leading dissidents have said they have faced even tighter restrictions on their movements in the run-up to Thursday’s anniversary.

Writer and social critic Jiang Qisheng, who was jailed for subversion in 1999 after he tried to organize commemorations on the 10th anniversary of the crackdown, said police had prevented him taking his wife to the doctor.

In Hong Kong, former 1989 student leader Xiang Xiaoji was blocked from entering the southern Chinese city after flying in with a US passport to attend a commemorative vigil on June 4. He was sent back to New York.

The wife of veteran dissident Hu Jia, who was jailed for subversion before the Beijing 2008 Olympics, was also blocked by police from leaving her home early Wednesday, London-based rights group Amnesty International said.