D.C. police have arrested a 22-year-old Northwest Washington man for allegedly raping a woman last month near Dupont Circle, and said last night they are investigating him regarding a recent series of sexual assaults that have alarmed neighborhood residents.

Police said plainclothes officers, relying on a sketch of a suspect made from descriptions supplied by residents, arrested Martin Eduardo Nunez of the 3100 block of 16th Street NW shortly after 8:30 p.m. Wednesday at 20th and O streets NW.

"It's been in a horrible state of siege around here," said Diane Bongiorni, who lives in the 2100 block of O Street. "One neighbor downstairs even left her apartment because she was so afraid to sleep there, and me and another neighbor have been sleeping on her living room floor for the past few days because it was the only room not accessible by a window."

Police said they charged Nunez with rape and burglary in connection with a Dec. 12 incident in the 2100 block of N Street NW. He is being investigated in connection with four other sexual assaults or attempted assaults in the Dupont Circle area and one in Mount Pleasant.

Three of the Dupont Circle assaults took place in December, and the fourth occurred Monday, police said.

"Police had targeted that specific area, and officers on patrol spotted a man who fit the composite sketch well," police spokeswoman Shannon Cockett said last night.

Police would not disclose details of the incidents, but neighbors interviewed last night said they were told by police detectives this week that a series of nearly identical sexual assaults or attempted assaults had taken place near the 2100 block of O Street NW since Christmas.

In each, neighbors said, the assailant apparently watched women as they entered their homes, and later that night climbed through an open window. Neighbors said police told them that in each incident there was no evidence of forced entry, and that the man had climbed through second- or third-story town house windows and escaped down fire escapes.

Another resident of the area, who declined to give his name, said he had printed 50 fliers this week describing the assailant and had planned to distribute them in the neighborhood.

"For the past week when I came home I always heard the sound of hammering," Bongiorni said. "Every night it seemed more people were nailing their windows shut."

Police said they had formed a task force to capture the assailant, and neighbors said nearby streets were "crawling with plainclothes officers" this week. Some neighbors said they were angry because police detectives did not walk door-to-door informing residents of the assaults until Monday.

Police said last night that they could not release more details on the assaults and their strategy to catch the assailant because their investigation has not been concluded.