Overall crime in the District rose 3 percent in 2012, with a significant rise in sexual assaults, even as the city recorded the lowest number of slayings in 50 years, Mayor Vincent C. Gray (D) and Police Chief Cathy L. Lanier said Thursday.

At a news conference, Lanier said that over the past four years the city has seen the largest decrease in killings among similarly sized U.S. cities. Eighty-eight people were killed last year, down 19 percent from 2011.

But the number of sexual assaults rose significantly, with 263 reported last year, compared with 174 in 2011. Most of the assaults involved acquaintances of the victims, Lanier said. About a third of the attackers were strangers.

Lanier said police are expanding prevention efforts by educating bar owners to be aware of the possibility of sexual assaults and by encouraging these owners to install better cameras and lighting.

She also praised the organization Men Can Stop Rape for helping curb sexual assaults by creating a smartphone app called “U-ASK,” which provides victims with support. Lanier said the app is being used by every college campus in the District.

Off campus, Lanier said, people can help prevent sexual assault by intervening if they see someone who appears to be intoxicated.

“Help them out, get them in a cab, get them home,” Lanier said, “Don’t let a stranger be the one to do it.”