OLD BRIDGE TOWNSHIP, N.J. (AP) _ A lonely widower who invited teen-agers into his bungalow for companionship had his home overrun by youngsters who used it for beer and marijuana parties, police said.

John Stief, 78, defended his young friends until they started assaulting him for his meager savings, police said Friday.

Three teen-agers and a 21-year-old woman were recently arrested on charges that they assaulted and robbed Stief and burglarized the small cottage where he lived, said police Detective Thomas Piscitelli.

''It's sad when teen-agers have to take advantage of elderly, defenseless people,'' he said Friday. ''But it happens much more than it's reported.''

After Stief's wife died nine years ago, loneliness overcame him, the detective said, and ''he thought it was quite innocent to invite local teen- agers in.''

Some of the teen-agers, however, were runaways who had been in trouble so frequently their parents refused to help them after their arrests, Piscitelli said.

''From what we understand, for about a year several kids were using his house as a party place,'' Piscitelli said.

''The first two girls never left his house,'' he said. ''They were constantly robbing him of his money, literally going right through his pants.''

Stief apparently realized the trouble, but the trap grew uglier.

''One time he tried to call the police and they ripped the phone out of the wall,'' Piscitelli said.

Stief's daughter, Evelyn Hatfield, would check on her father several times each week, bringing groceries. She suspected something was amiss, but did not call police until she visited and found her father asleep in a back room while almost 20 teen-agers held a party in the front of the cottage, Piscitelli said.

''One day at the beginning of October, we went to check on him and he opened the door only partially and didn't invite me in,'' Piscitelli said. ''I knew something was going on and persuaded him to let me in. We found two girls hiding in the bedroom and found out they had forced their way in.''

The girls subsequently were arrested and charged with burglary and theft. Because the girls, ages 15 and 17, are juveniles, police are withholding their names. They are being held at the Middlesex County Youth Detention Center.

The other two arrests were last week, Piscitelli said, ''because now he (Stief) is starting to fight back. He's a very fragile man, and while one girl held him, the other went through his dresser drawer and took $35.''

Stief called police, who investigated the theft and eventually arrested another 17-year-old girl and 21-year-old Debra Heinen of Keyport. They were charged with theft and assault, Piscitelli said.

''They admitted to police that they went out and bought beer with the money and had a good time,'' he said.

When police questioned Stief about the thefts, Piscitelli recalled, ''he broke down in tears and said, 'Look, I don't have nothing to take.' ''

On Friday, Stief refused to discuss the thefts and assaults, saying, ''I don't want to talk about it. I just want to forget it.''

When reached by telephone, Mrs. Hatfield refused to talk to a reporter.

Piscitelli said police tried to get Stief involved in the local senior citizens groups, ''but apparently about nine years ago, he and his wife participated and he felt he got the cold shoulder.''

Dolores Lastoczy, program supervisor for Old Bridge Township's Office On Aging, said some people just will not participate in organized activities.

''The children bring their parents in and if you get them on a bad day, they don't want to stay,'' she said.