When John W. Fraber's Dale City storefront window was smashed for the fourth time this year, he was furious.

When the vandals then stole the barricade he put up in front of the broken window, he chased them at gunpoint into the woods near his handicrafts store while a neighboring merchant called the Prince Williams County police.

Twenty minutes later Fraber backed off from the two bottle-wielding youths and let them go because the police had not arrived.

Yesterday Fraber joined 70 citizens who demanded county supervisors add 30 officers to the 151-member police force because they said it's too small to adequately protect residents in the sprawling county that's four times the size of the District of Columbia.

The group, mostly from Woodbridge and Dale City in eastern Prince William, complained that response time for crimes in progress can be as long as 15 minutes to an hour and that other calls often are not answered at all.

Since he was vandalized, Fraber said he has also been robbed in his store and that a nearby merchant had been robbed twice in the last two weeks. Police took more than 20 minutes to arrive in each case, he said.

Elizabeth Connors, another Dale City resident, complained yesterday that police didn't respond for more than an hour and then refused to make any arrests when more than 100 teen-agers staged a "drunken orgy" on her subdivision street on Aug. 31.

County Police Chief George T. Owen acknowledged the force's response time has increased. He said officers have been required to handle more investigations and often must travel to the western portion of the county to obtain arrest warrants. Frequently there is no magistrate available in the populous eastern section of the county, he said.

However, the chief said that he has no figures on how severe the response problem is. "It's more of a feeling I have that response time has increased. I haven't specifically analyzed it," said the nine-year veteran of the force.

Only 87 of the 151-member force are assigned to patrol duty. Residents complained yesterday that because of shift changes and lack of officers, only one unit patrols the Dale City-Woodbridge area after midnight.

Prince William County, with a population of 145,000 ranks second lowest in Northern Virginia for the number of police per capita. Loundoun County has .86 officers per thousand, compared to Prince William's .96 per thousand resident.

Fairfax County, with the largest police force, has 1.21 officers per thousand. Alexandria with a population of 116,800 has two officers per thousand.

Owen told the supervisors he could not handle an increase of more than 20 officers per year because each new recruit would have to be investigated and trained.

Supervisors had recommended only an increase of two officers for 1980 based on last year's budget projections. The force has increased by 16 since 1977.

Supervisor James J. McCoart, who represents the Dale City-Woodbridge area of the county, called Owens' response to the citizens "totally unacceptable."

McCoart said the county has received more than $12 million more in revenue than it predicted last fall and could add the 30 officers for less than $500,000.

The residents, who had shown up in force at a board work session, were not given the chance to speak, cornered chief Owen for a two-hour session instead.

Later, after McCoart reported the group's concerns during the board's regular meeting, the board agreed to hear the complaints at an Oct. 2 meeting to be held in the Dale City area.