On a recent evening, Alphonso Prater saw two young men arguing on a street corner about a woman. Prater knew the guys from the neighborhood and realized that if he didn't step in, someone could get shot.

"I told them: 'Neither one of you is going to win. If you hurt him, then you're going to jail. And then your woman will just get with someone else,'" he said.

The two men eventually calmed down and went their separate ways.

Prater, a 49-year-old ex-convict, intervened in his role as a "violence interrupter" for CeaseFire, a nonprofit program that tries to prevent shootings.

Its effectiveness is hard to measure with precision, but some law enforcement authorities, civic leaders and politicians credit CeaseFire with contributing to the drop in Chicago's homicide toll to its lowest level in decades.

CeaseFire has about 20 violence interrupters in Chicago. Many are former gang members and ex-convicts. All know the streets and those who run them. They drive around and walk door-to-door in some of the city's toughest neighborhoods to see what is going on and try to defuse arguments before someone pulls the trigger.

"Some people can go on the front porches and talk to people, but these guys can get into the inner circles. They get inside the front doors," said Tio Hardiman, a former street-hustler-turned-academic and CeaseFire's director of gang mediation services.

The program, funded by state and federal governments and various foundations, debuted in Chicago's West Garfield Park section five years ago and is now in 15 neighborhoods, with a $5 million budget last year. It is spreading to other cities in Illinois and around the nation, including Newark, N.J., and Baltimore.

"Law enforcement catches you after you cross the line. CeaseFire keeps you on this side of the line," said founder Gary Slutkin, an epidemiologist at the University of Illinois at Chicago.

CeaseFire — which also holds rallies, hangs up anti-violence posters and works with clergymen and community groups — is trying to change the mind-set of many who have grown up around violence.

Prater, who has been in prison for drug offenses, said he has seen too many disputes about a girlfriend or a dice game escalate into gunfire in his West Side neighborhood.

"Some people don't know how to fight. They just get a gun. And I just don't want to see them messing up their lives," he said.

Chicago had 641 homicides in 1999, the year before CeaseFire began. By 2004 that number had fallen to 448. Last year, there were 447 killings — the first time the city recorded two consecutive years with fewer than 500 homicide since 1964-65.

Though Chicago police say their attack on gangs, drugs and guns is largely responsible for the drop in killings, they give some credit to CeaseFire. In neighborhoods where the program operates, homicides dropped by nearly half, from 97 in 2003 to 49 in 2004, according to CeaseFire.

"They work directly with the community groups, and that structure makes it unique," said Police Deputy Superintendent Charles Williams. "They don't just go to the community meetings and say, `We're here for you.' They also are walking the neighborhoods."

In May, the program began in a two-square-mile area in Newark, N.J., and adjoining Irvington. Since then, shootings are down 30 percent compared with the rest of Newark, said Mike Wagers, Operation CeaseFire's director there.

Marnell Brown, a CeaseFire worker in Chicago, said he and other violence interrupters are on call 24 hours a day.

"You may not always want to get up when you get a call, but you do it because it's important," said the 47-year former gang member. "It's hard, but this is where my heart is. I took from the community, and I know now I'm giving back. That's a good feeling."