Chinese authorities are imposing severe security measures in central Beijing today as the city passes the 25th anniversary of the Tiananmen Square massacre.

Thousands of police and security forces, some armed with automatic rifles, have been deployed across the Chinese capital.

Police trucks are reportedly stationed near Tiananmen Square, alongside fire engines and ambulances.

Tourists are still able to move about the vast public square, although uniformed and plain-clothes officers have reportedly been demanding identification from passers-by.

Reporters have also been told to leave the area following the daily crack-of-dawn flag-raising ceremony.

Soldiers stand guard near Tiananmen Square in Beijing

Dozens of activists and other government critics have already been detained by police in the lead-up to the anniversary.

Hundreds of protesters demanding democratic reform were killed when soldiers opened fire on crowds gathered in the square in 1989.

Some estimates put the death toll from the massacre at more than 1,000 people.

The crackdown saw Chinese soldiers crush months of peaceful protests by students who were demanding political reforms.

Government censorship means that many Chinese people know very little about what happened during the crackdown.

The US has used the 25th anniversary to call on Beijing to increase political freedom in the country and release political prisoners.

 'Tank Man' - the image that came to symbolise the Tiananmen Square protests and subsequent bloody crackdown

"We've very clearly called on the Chinese authorities to release all the activists, journalists and lawyers who have been detained ahead of the 25th anniversary,” said State Department spokeswoman Marie Harf.

"I think it's time to allow some more space, quite frankly, for discussion in their own country, particularly around this kind of anniversary."

Around 150,000 people are expected to attend an annual candlelight vigil in Hong Kong to commemorate the victims of the massacre.

Tens of thousands of Hong Kong residents congregate in the city's Victoria Park every year for a solemn ceremony to mark the events of June 1989.

This year's event is expected to draw unprecedented numbers to the vigil, including an ever-larger cohort of mainland Chinese who travel to Hong Kong.