China has detained dozens of people and launched an unprecedented security operation in central Beijing to stop anyone commemorating the 25th anniversary of an event that has been virtually wiped from the collective memory of the nation.

A quarter of a century after the People’s Liberation Army turned its tanks and guns on the people and marched into Tiananmen Square on the night of June 3 and early hours of June 4, 1989, any mention of the massacre of hundreds, perhaps thousands, of people is banned from public life.

More than 50 people, including activists, lawyers, journalists and relatives of students killed in the massacre, have been detained, arrested or simply “disappeared” in recent weeks because of their efforts to commemorate the anniversary, according to human rights groups.