Several crews of teenagers who police say are responsible for about 100 armed carjackings and robberies in the border region of the District and Prince George's County this year have been arrested over the past two months, authorities reported yesterday.

In a major offensive against crime along the border, more than 30 teenagers, ages 14 to 19, were arrested by city and county police. As a result, police said, the number of carjackings in Prince George's has dropped by 70 percent in recent weeks.

In the first two weeks of October, there were 60 carjackings in Prince George's. That number dropped to 18 in the first two weeks of this month, according to police data.

Before this month, an average of 15 carjackings occurred each weekend this year in Prince George's. This past weekend there were none, according to police.

"This is a direct result of the arrests," said Maj. Vincent Gay, head of the Criminal Investigations Division. "We believe there's a small group of people committing the great majority of the crime."

Robberies, especially carjackings, are a vexing public safety problem in the county. There were more carjackings in Prince George's last year than in the rest of Maryland's communities combined.

So far this year, the number of carjackings is up almost 60 percent from the same time in 2004, and the number of robberies has jumped by 23 percent. Since January, almost 700 carjackings have occurred in Prince George's, and about 190 happened in the District.

Carjackings in the District are running at about the same rate as last year, statistics show.

Police say the four crews arrested recently committed at least 50 carjackings and 40 robberies from January through October. The home addresses of the suspects are mostly in the District; some are in Prince George's.

Several other people who police believe are involved in the crimes have not been caught, they said.

Criminals often use the border to their advantage, slipping from one side to the other to avoid capture. Police in both jurisdictions began sharing information more intensively this year to attack the problem.

"Criminals should realize that the border between the county and the District is no longer a haven for them," said Lt. Sean Carney, commander of the Prince George's robbery section.

Those committing the crimes are usually armed and often are teenagers who approach someone getting into a car. The crimes happen at gas stations, at shopping centers and in front of people's homes, Gay said.

"We're seeing teens heavily involved in the area of carjacking and street robberies," he said. "The appeal is getting nice vehicles from the victims. We're being told it's mainly for joy riding. They ride around until they are stopped by police or until they pass [the] vehicle on to somebody else."

Gay said Prince George's police created a carjacking unit at the beginning of this year to tackle the problem. Last year, 563 carjackings occurred in Prince George's; there were 492 in the rest of the state.

To avoid being carjacked, Gay said, drivers should be acutely aware of their surroundings, especially at night. He advised parking in well-lighted areas that are not isolated.

A driver who senses something is suspicious is probably right and should drive to a populated area and call police, Gay said. "We need information," he said. "We need feedback from the community."