Two days after three Yale students were victims of an armed robbery in East Rock in the middle of the afternoon, Yale Police Department Chief James Perrotti announced on Sunday the deployment of a “special street crimes unit” that will respond to the increase in street-level crime in the neighborhood.

The unit, which will consist of four officers and one supervisor, will aim to help New Haven Police Department officers combat the rise in robberies affecting Yale graduate students living in East Rock. Community leaders from the area said they have been concerned for weeks about the apparent uptick in crime in the otherwise relatively safe neighborhood. A few have even attempted to supplement the extra police presence by creating a nighttime block-watch group.

In the most recent robbery, an armed male robbed three graduate students Friday on Humphrey Street in East Rock at 12:58 p.m. The robbery occurred close to the New Haven Lawn Club, a popular venue for swanky Yale events, although two members of the Lawn Club’s staff said Friday afternoon they were not aware of the robberies.

On Sunday, the NHPD arrested 29-year-old New Haven resident Michael DeMartino, who police claim is responsible for several recent East Rock robberies, including Friday afternoon’s, according to a statement issued by City Hall Spokeswoman Jessica Mayorga.

Several East Rock residents, including Ward 9 Alderman Roland Lemar, said the Friday afternoon mugging likely reflects a recent upswing in “violent crime” in the streets of that neighborhood. Lemar said it is usual for crime to increase during periods of increased moving activity — start of fall term, start of winter recess and end of spring term — but he said he realizes the latest bout of East Rock crime suggests that “crime has taken an aggressive turn.”

Residents said their concerns began Aug. 21, when an armed male shot a female graduate student in the hand while trying to steal her wallet. The crime occurred on the corner of Edwards and Nicoll streets in East Rock.

The deployment of the YPD’s “special street crimes unit” will augment increased NHPD patrols in the East Rock neighborhood, Perrotti wrote in an e-mail to the Yale community Sunday. The YPD has deployed the special unit in other New Haven neighborhoods before, but is being introduced to East Rock for the first time, he said.

In addition to the expansion of the YPD’s street-crimes unit, civilian block watches have been engaging in increased community patrolling. David Streever, an East Rock resident, established the latest of these crime-watch groups three weeks ago. His group, called East Rock Neighbors, aims to keep local streets safe and hospitable to residents, Streever said.

The East Rock Neighbors group, which currently has approximately 30 members, patrols the neighborhood on foot every night in order to increase neighborly visibility and discourage criminals from targeting East Rock residents, he said.

“We’re trying to foster a sense of community,” Streever said. “We want to encourage people to walk around the neighborhood at night — in groups.”

Ben Berkowitz, another member of the East Rock Neighbors Group, said Yale students living off-campus in East Rock are very involved in the crime-patrol group. One-third of the group is composed of Yalies — contrary to popular belief about Yale students’ indifference to community crime affairs, Berkowitz said. He said he thinks Yale students living in East Rock think of themselves more as East Rock residents than Yale students.

“There’s a tremendous stereotype about naive Yalies being the victims of crime in East Rock,” Berkowitz said. “But all of us are partially victim to these crimes.”