Beijing launched armed police patrols Monday to handle violent incidents in the capital, city authorities said, three weeks ahead of the 25th anniversary of the Tiananmen crackdown and after a series of attacks on civilians.

The armed division, which has 150 vehicles, aims to "strike on terrorist and violent crimes in a rapid and effective way", the Beijing Municipal Public Security Bureau (PSB) said on its verified account on Weibo, a Chinese equivalent of Twitter.

The patrols were being deployed in the face of "the current severe and complicated situation of anti-terrorism," it said.

The Chinese government is on alert after a series of dramatic attacks it blames on militants from far western Xinjiang, home to the mainly Muslim Uighur minority.

A fiery vehicle crash in Beijing's Tiananmen Square -- the symbolic heart of the Chinese state -- last October was followed by a horrific knife assault in March at a railway station in the southern city of Kunming, which left 29 dead and 143 wounded.

Two weeks ago, assailants using knives and explosive devices attached to their bodies attacked a train station in Urumqi, Xinjiang's capital, leaving three dead -- including two alleged attackers -- and 79 wounded.

The Beijing PSB said each patrol vehicle will have an armed team of at least two police officers and one auxiliary on every shift, who will arrive on the scene within three minutes of a report.

They will park at main roads, crowded venues and unspecified "key areas" -- possibly a reference to politically sensitive sites such as Tiananmen Square -- and set up checkpoints, it said, adding: "They will respond immediately whenever an emergency takes place."

State-run media posted pictures of black Hummer-like vehicles online.

The announcement came three weeks ahead of the sensitive June 4 anniversary of the brutal suppression of huge anti-government protests in Tiananmen Square in 1989, when hundreds -- by some estimates, thousands -- were killed.

Authorities routinely round up outspoken critics of the Communist Party in the weeks before key events and police have recently detained prominent former journalist Gao Yu, celebrated human rights lawyer Pu Zhiqiang and other activists.