A thunderous blast outside Dongzhimen Sky Plaza in downtown Dongcheng district in Beijing injured an American student Thursday afternoon.

A policeman collects debris at the junction of Dongzhimenwai Dajie and Dongzhong Jie Thursday. Photo: IC

There were no other injuries, police said, and the 30-year-old was taken to the PLA Military General Hospital with a slight leg wound.

"The sound of the explosion was so massive I estimate people within a 10-kilometer radius heard it," a security guard at the Dongzhimen subway station entrance C told the Global Times.

"I saw gray smoke rising after the explosion and I smelled gunpowder in the air. It was probably a bomb," said the guard, an ex-army soldier, on condition of anonymity.

"Police were dispatched to the scene immediately and they were looking for other bombs, I think."

Smoke rose five floors high, according to the China Network Television.

It felt like a "firecracker exploding under my feet," said an employee surnamed Ren who works on the sixth floor of Sky Plaza (Tianheng Dasha).

"I heard the sound and thought: 'Is the building falling apart?' Then I guessed it must be something inside the building going wrong."

"I never thought it was an explosion from outside the building," added her colleague surnamed Wang.

A witness 30 meters from the explosion "felt ringing in the ears and buzzing inside his head," China Network Television reported.

The 3:10 pm blast had originated from bushes beneath a street lamp in the parking lot outside the China Merchants Bank and charred the plastic of a news kiosk a few meters away, a source told the Global Times on condition of anonymity.

Police troops, armed police, special police, firefighting troops and ambulances all rushed to the scene within an hour of the explosion.

The blast zone had been sealed off by two security cordons by 5:30 Thursday. Officers were taking photos and collecting fragments in transparent plastic evidence bags at the scene.