Tens of thousands of Beijing commuters are being forced to queue for up to an hour morning and night by stepped-up subway station security imposed this week after several deadly attacks and ahead of the June 4 Tiananmen crackdown anniversary.

China's capital is already notoriously crowded, with a population of 20 million people, and lines outside some stations stretched for dozens of metres at rush hour.

At one suburban station the queue snaked up and down between metal fences taller than the people between them.

"It's frightening to think what a terrorist could do to this crowd of people," said one user of microblogging website Sina Weibo.

Beijing has long had hundreds of X-ray machines and security staff stationed at subway station entrances to check passengers' personal belongings, part of a series of measures imposed ahead of the 2008 Olympic Games and never withdrawn.

The new precautions in force at many stations -- but not all -- require passengers to undergo full-body security checks.

Armed police are also being dispatched throughout the city, part of "a three-tier patrol protocol that covers the skies, subways and streets", the state-run China Daily newspaper said Wednesday.

June 4 marks 25 years since the 1989 crackdown on pro-democracy protesters in Tiananmen Square, one of China's most politically sensitive anniversaries, and authorities routinely clamp down on any attempts to commemorate it publicly.

China has also been grappling with a spate of violent attacks targeting civilians, which Beijing has blamed on separatists from the far-western region of Xinjiang.

Last week five attackers killed 39 people and wounded more than 90 at a market in Xinjiang's capital Urumqi.

In March knifemen killed 29 people and wounded 143 at a railway station in the southwestern city of Kunming, an incident dubbed "China's 9/11" by state media.