"There was lots of debris at the scene. Police swept it away," said an onlooker.

An officer climbed atop the kiosk and swept shrapnel off the roof as a bemused vendor remained inside the booth, guarded by dozens of police.

At 6:30, the road in front of the Sky Plaza was still congested by police vehicles and curious onlookers. Police officers and security guards ordered onlookers to move along.

"Taking photos is forbidden here," said a security guard at the Sky Plaza when the reporter approached.

At least 15 police vehicles and three special police vehicles were still at the scene 6:30. Their vehicles occupied about 200 meters of the west-east Dongzhimenwai Dajie.

Several bus stops fell inside the cordon, forcing commuters to go to the next stop. Dongzhimen is a transport hub, meaning traffic and public transport was affected throughout the city.

Police refused to speculate on the cause.