Police say traffic cameras frequently capture images of motorcycles and souped-up cars dragging along elevated roads. But most drivers use fake license plates when racing so camera images rarely lead to arrests.

Most of the racing takes place late at night or early in the morning when there are few vehicles on the roads, police said. They added, however, that some drivers prefer to race around noon, assuming there will be few police on the streets as shifts are changing or officers are having lunch.

"We must take timely actions to stop this illegal activity when it has just begun to become popular," said Guo Ji, an officer with local elevated road traffic police.

Some drag racers have even taken to the Internet to brag about their adventures.

After CCTV broadcast a report recently about the prevalence of racing in Beijing, some devoted underground racers in Shanghai posted messages on the Web, calling on drivers to promote drag racing in the city.

One poster stated: "In no way should Shanghai have weaker drag-racing groups than Beijing does."

Guo said the races pose a huge risk to everyone on the road as cars often hit speeds of 150 kilometers an hour, and motorcycles zip by at 120 kph or faster.

Police said the activity tends to attract more motorcycle riders than car owners. Motorcycles aren't allowed on elevated roads, but that doesn't seem to deter the racers.

Police said the most of the racing vehicles were souped up with more powerful engines and exhaust system.

On several occasions police have tried to stop racers only to watch them speed off, paying no attention to the officers. Police sometimes find themselves chasing both cars in a race, which often split up to make capture more difficult.

"For a single patrolling officer, it's difficult to catch both the racers in the chase. In a case last year, one of the motorists even sped over three road blocks before he was finally seized." Guo said.

A traffic police officer, Ding Jianzhong, suffered injured knees when trying to grab a racing motorist on Huqingping Overpass last summer. The vehicle hit his knees when the motorist tried to escape.

Once caught, racers will generally lose their licenses and police will detain their vehicles.

A refitted Sail drove more than 150 kilometers an hour and crashed into a guard rail on the Middle Ring Road near Yixian Road last July. Rescuers had to cut the injured driver and his female friend out of the twisted wreckage.

Police suspected the car was engaged in a drag race, but the driver denied that charge.