SHANGHAI - Police in Shanghai cracked more than 20,000 criminal cases in the first quarter of this year as a result of beefed-up security for the Expo, the city's public security bureau officials said on Tuesday.

More than 15,000 suspects were detained in a series of raids and operations, according to police. And the city's first-quarter crime rate dipped 30 percent compared to the same period last year.

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The police's undercover squad has caught more than 2,000 suspects and broken 160 organized criminal rings in more than 4,000 property violation cases this year.

During the two weeks prior to the Expo from April 15 to 29, some 4,500 police officers and 7,500 armed police jointly patrolled the city's streets 870 times in vehicles, in a bid to deter criminals on the city's streets.

Nearly 50,000 community security guards and volunteers participated in the citywide patrols, checking more than 630,000 people on the streets who looked suspicious and seizing 21,000 hazardous objects since April 15, according to the public security bureau.

The police said no serious criminal cases had happened in the city during the Expo's six-day soft opening and four days of formal operation until Tuesday.

Some 850,000 Expo volunteers are now in the city, mainly college students and retired citizens, mobilized by the police to provide information services to outlanders who are not familiar with the megacity and deter potential law-breakers.