TROY, Ohio (AP) _ Police in riot helmets dispersed dozens of demonstrators in the third night of protests against the killing of a teen-ager by two police officers, and 24 people were arrested, authorities said today.

About 100 people, mostly teen-agers and young adults, gathered in the city's downtown Public Square after Monday night's City Council meeting, said Miami County Sheriff L.B. Dunfee.

At the meeting, residents had demanded answers to questions about the June 9 shooting of 18-year-old Kerry Helton. Police have said the two officers fired six shots each at Helton as he advanced toward them with a knife after the officers kicked in the door to a bedroom in his mother's home.

The protesters, demanding the dismissal of the two officers, blew horns and some threw bottles, and police warned them they had three minutes to leave the square or be arrested, Dunfee said.

Twenty-four people were arrested, most for disorderly conduct, the sheriff said. Three people were charged with misdemeanor rioting; one was ordered held on $1,000 cash bond, the second was released after posting bond and the third person, a juvenile, was released to his parents, authorities said.

Four other people were arraigned in Municipal Court today for probation violations, said deputy court clerk Shanda Boehringer. Most of the others arrested were given citations, which are similiar to traffic tickets, and were not jailed and would not be arraigned, she said.

Dunfee said he was not aware of any injuries.

More than 300 people demonstrated Sunday night in the western Ohio city of 18,000. Several citations were issued for such offenses as littering, excessive horn-blowing and jaywalking, but no arrests were made, police said.

On Saturday night, four people were arrested and a number of traffic citations were issued after protesters, many of whom were friends of Helton, drove their cars around Public Square, honked their horns and tied up traffic.

At Monday's council meeting, Council President Peter Jenkins said the body would listen but would not answer questions about the shooting because two investigations of the shooting have not been completed.

Jenny Short, 18, one of about 30 people who attended the meeting, said the demonstrators ''understand we don't know all the facts. At our age, all we can feel is rage. But we're not a bunch of crazy kids who want to humiliate the police department.''

Miami County Prosecutor Jeff Welbaum said Monday that he and two prosecutors from nearby counties would investigate the shooting.

The prosecutors are expected to begin presenting evidence to a county grand jury July 15, and it will be up to the grand jury to determine whether criminal charges are warranted against Officers Steve Cruea and James Fox.

Troy Police Lt. Jack Green also is conducting an internal police investigation.