Chinese authorities are going to great lengths to ensure that today's 20th anniversary of the brutal Tiananmen Square crackdown on students is not marked by protests.

Twenty years ago today the might of the world's largest army was unleashed on several thousand students who had been protesting for more than six weeks in support of democratic change.

Tanks, armoured personnel carriers and thousands of troops crushed and shot anyone who got in their way, and to this day the Chinese government has concealed the exact death toll.

Most of the deaths were in the streets around the square where tens of thousands of workers and ordinary citizens attempted to stop the armoured columns.

Today the area is occupied by thousands of soldiers and police ready to prevent any commemoration of the historic events.

Uniformed and plain-clothed officers are patrolling the square, while heavily-armed paramilitary police carrying automatic weapons stand nearby.

There are no signs of the occasion being marked anywhere, and any people the government feared would hold memorial services or protest have been put under house arrest.

News reports coming into China about Tiananmen have been blocked, as have social networking websites like Twitter, for fear they could be used to organise protests.

Meanwhile, the United States government has called on China to release the names of all who were killed or went missing.