At least 10,000 people are believed to have been killed during the Tiananmen Square massacre in Beijing in 1989, according to a newly released British diplomatic cable.

The document has been made public 28 years after troops brutally clashed with students at a pro-democracy demonstration in 1989.

In the cable, the British ambassador to China at the time of the massacre, Alan Donald, recalls seeing injured girls being killed with bayonets, people being mown down by bulldozers and their remains being flushed down street drains.

"Minimum estimate of civilian dead 10,000," the telegram reads, AFP reported after viewing it at the National Archives in Kew. This estimate is almost 10 times higher than the accepted death toll.

Experts say that the new estimate seems credible, with China scholar Jean-Pierre Cabestan commenting that it was "not particularly astonishing considering how crowded it was in Beijing."

In the cable, Donald says he witnessed armoured personnel carriers (APCs) who "opened fire on the crowd (both civilians and soldiers) before running over them in their APCs."

He says that once the soldiers arrived at the square, "students understood they were given one hour to leave square, but after five minutes APCs attacked."

"Students linked arms but were mown down, including soldiers. APCs then ran over bodies time and time again to make 'pie' and remains collected by bulldozer. Remains incinerated and then hosed down drains," the telegram reads.

"Four wounded girl students begged for their lives, but were bayoneted," it continues. "Army ambulances who attempted to give aid were shot up."

The massacre was carried out by the 27th Army, according to Donald. He describes the soldiers as "60 per cent illiterate and primitives."

Almost three decades after the democracy crackdown, the Chinese government continues to ban schools and the media from mentioning the Tiananmen massacre.