PHILADELPHIA — Philadelphia announced Monday a nighttime curfew for minors in its historic downtown in an attempt to stop flash mob invasions by rampaging teenagers.

The measure was the latest effort to secure a city famous as the site of the signing of the US Declaration of Independence and drafting of the US Constitution, but which has spent years trying to overcome violent crime.

Starting Friday, anyone under 18 will be subjected to the 9:00 pm curfew Friday and Saturday nights. Teens caught violating the curfew could be fined between $100 to $300, while parents get a warning for the first violation and face up to $500 in fines for subsequent violations.

"Let this be a message to any others who think that participating in flash mobs is acceptable or fun -- don't do it," District Attorney Seth Williams said.

"We will apprehend you, prosecute you and send you away. You will not damage the reputation of our great city."

Often organized on networking sites like Facebook and Twitter, the teen rampages bring vandalism and assaults, resulting in everything from broken bones to stabbings in what city officials have tried to make a haven for tourists and businesses.

The curfew affects the Center City neighborhood, home to a vibrant arts and restaurant scene and many of Philadelphia's major historical attractions, as well as University City, near two of the city's major colleges.

Officers may send the minors home, drive them home or call their parents to pick them up at a police station. Parents who do not immediately pick up their child could see themselves investigated for child neglect.

In other parts of the city, a 10:00 pm curfew remains in effect for minors under the age of 13 and 12:00 am for those under 18.

Philadelphia Mayor Michael Nutter had a stark message for troublemakers as he vowed to put a stop to the violence in the heart of the city.

"You are out wreaking havoc. You are out causing problems. You are out doing things you should not be doing. We are going to get you off the streets," he said.

The most recent attack occurred July 29, when a crowd of 20 to 40 youths beat and robbed two people in an episode that spanned two city blocks and continued unimpeded by the police.

The beatings, which left at least one man unconscious and another requiring surgery to his jaw, took place in the shadow of Philadelphia's iconic City Hall, and a block from the Avenue of the Arts housing the city's orchestra and theaters. Four people were arrested, including an 11 year-old boy.

In June, 50 to 100 youths attacked several people, including the editor of an alternative weekly, who suffered a broken leg.

The beatings have put businesses in the area on edge.

"There's nothing good you can say about them," said Paul Levy, executive director of the Center City District, a group working to improve business conditions in the tony neighborhood.

"These are random acts of violence that occurred because people thought they could get away with it."

Dozens of new restaurants and arts institutions have led the way to a 20 percent increase in residential population over the past decade in Center City, and the neighborhood is among the trendiest in the city.

For locals like Donna Wallace, a commercial loan specialist who works in the city but lives in the suburbs, the flash mobs are a frightening phenomenon, keeping her off the trains if the streets are empty.

"I definitely wouldn't want to stay downtown now," Wallace said. "I leave with the crowds."

City officials have struggled to respond. After attacks last year, a city councilman asked the mayor to sue Facebook and Twitter.

Police have beefed up patrols in the neighborhood and asked residents and businesses to register their security cameras with the authorities.

Some young people fear that the attention given to the flash mobs could have negative repercussions for those not involved.

Travis Holmes, 21, worried that the police response will give authorities more license to arrest young, African American men.

"You can just be walking down the street by yourself and they'll stop you," Holmes said. "But as young African Americans, you can't target all of us."

Tasha Jamerson, a spokesperson for the Philadelphia District Attorney's Office, said there was not much that could be done under current laws to stop the problem.

"You are limited when prosecuting juveniles. With some of those kids, all you can get (for punishment) is community service," she said.