The District has seen some crimes, including homicides and robberies, fall in 2011, Mayor Vincent C. Gray and Police Chief Cathy L. Lanier announced Wednesday.

Homicides were down 11 percent and robberies down 7 percent through Aug. 1, Lanier said.

In a joint news conference at the Mary Virginia Merrick Center in Southeast, the chief said that violent crime fell by an average of 37 percent in five neighborhoods that historically have had persistent crime problems.

However, the statistics released Wednesday show that total crime in the District, including violent crime and property crime, was up by 3 percent through August.

Gray (D) attributed the decreases to a summer effort to address such issues as abandoned cars, overgrown grass and youth crime.

Beginning in May, Gray said, he directed city agencies to work together so that quality-of-life problems — including dirty playgrounds, dark alleys, and overgrown grass and weeds — were addressed “within hours.”

Additionally, he said, residents have taken part in programs such as city-sponsored basketball and football games.

Gray said the city’s summer jobs program has also helped keep young people safe. Nobody under the age of 18 has been killed in the District since June 21, he said.

Gray said that the collaborative strategy will continue year-round and that a report on the effectiveness of the summer effort will be released in the fall.