A brazen serial prowler targeting homes in Alameda, including one where he sexually assaulted a 7-year-old girl as she slept in her bed, has left residents of the island community on edge and police beefing up patrols to catch the culprit before he strikes again.

The series of incidents has happened in the predawn hours in the west end of Alameda. Police believe one man, who was captured on a local motel’s surveillance video, is responsible for all of the attacks.

At least six incidents have occurred since June. While several residents have been accosted in their homes, fear throughout the community increased when the prowler entered the bedroom of the 7-year-old girl Sunday night and sexually assaulted her.

“We’re concerned about it, of course. Law enforcement can only do so much,” Duke Hernandiz, 77, said Tuesday, as he walked with his wife Roslyn on Linden Street near Haight Avenue, near the site of one of the break-ins. “We’re very vigilant about who’s around the neighborhood.”

Hernandiz said he counts himself among those who keeps his doors and windows “always secured.”

Uniformed and undercover police officers have been working around the clock as city officials and residents hope the spate of break-ins and the assault will not tarnish the image of the community of 76,000 that prides itself as being the “city of homes and beaches.”

Police are warning residents to not keep their windows open or doors unlocked, something many have felt comfortable doing before the crime wave, especially in the hot summer months.

“The weather’s been warm, so it’s been tough, because people want to get more air flow,” said police Lt. Jill Ottaviano. “But you have to secure your house.”

Alameda police say they are following up on a number of leads and are scouring sex-offender registries. They have also upped patrols and deployed undercover officers.

Police have contacted local businesses, the city’s Housing Authority and the school district to see if anyone can identify the young man in the video, Ottaviano said.

Authorities say they don’t know if the man is targeting his victims, watching or following them or selecting homes at random.

The man has entered or tried to enter homes between midnight and 7 a.m., often climbing through unsecured windows.

The first incident happened at the Rodeway Inn along busy Webster Street on June 16, when the man broke into the manager’s unit through a window while the manager and his family were inside. Security cameras caught images of the prowler on the property before he was scared off.

The man captured in the surveillance footage is dark-skinned, at least 18 years old, weighing 150 pounds, with hair that is curly on top and short on the sides.

In the ensuing months, other break-ins were reported on the 1500 block of Linden Street, the 500 block of Central Avenue and the 300 block of Tucker Avenue.

In the most recent incident Sunday, a woman called police to report that an intruder had sexually assaulted her 7-year-old daughter sometime overnight as the child slept in her bedroom at the sprawling Summer House apartment complex on the 400 block of Buena Vista Avenue, police said.

The attacker got into the home through an open bedroom window after cutting the screen, police said.

In another case, the man stole a car. Each time, he disappeared very quickly, leading police to suspect he lives nearby and is familiar with the west end of the city.

“It sounds like there’s a very emotionally damaged person out there wreaking harm on others,” said Grover Wehman as she walked with her two children, ages 2 and 4 months, on Linden Street. Still, “I don’t feel unsafe walking in Alameda,” Wehman said.

But a 20-year-old woman who wished only to be identified as Rebecca said, “I feel really unsafe to leave my window open at night.”

Rebecca, who spoke as she walked with her 2-year-old son near the Summer House apartment complex, added, “Since it’s Alameda, people typically leave their doors unlocked or their sliding glass doors open at night, because it’s hot in the summer, and now you can’t do that.”

Mayor Trish Spencer is asking community members to be vigilant,

“The community, as well as definitely our police, our City Hall, we all take it very seriously,” Spencer said. “And so we are all stepping up, and hopefully they ascertain (who the suspect is) and bring this to a close.”

Anyone with information is asked to call Alameda police at (510) 337-8340.

Henry K. Lee is a San Francisco Chronicle staff writer. E-mail: hlee@sfchronicle.com Twitter: @henryklee