A week ago, four white juveniles and an adult allegedly stole a fire extinguisher from an apartment house and drove along Layhill Road in Wheaton, spraying 12 blacks with its caustic contents before police arrested them.

On May 26, tar was smeared on the door of a school in Wheaton and set afire, according to Montgomery County police. On the door, below a Jewish prayer cap, was written: "Jew school, wear your beanie!"

In April, an Oriental family in Silver Spring, whose car and home windows had been shattered in two previous incidents, awakened to find the words "Chink Face" and "F--- You" scrawled on the rear of their house, police said.

Both Montgomery police and the county's Human Relations Commission have noted an increasing number of racially and riligiously motivated incidents in traditionally liberal Montgomery County.

Police say more than 30 such incidents of cross burnings, harassment, vandalilsm and assults have been reported this year; there were 22 reported in all of 1980.

"This kind of attack has the inevitable effect of unnerving thousands of people, not just the 'prime targets'" of each incident, said Montgomery County Executive Charles W. Gilchrist in a letter to State's Attorney Andrew Sonner last Wednesday, "These are not pranks. . . These are savage assaults on the sensibility of our fellow citizens, primarily those who are Jewish or black." Gilchrist said he is confident the prosecutor's office will transmit the "unmistakable message that this kind of conduct will not be tolerated in Montgomery County."

To some local blacks leaders, the increase parallels deteriorating race relations across the country. Others say it simply means that "closet racists," who they say went underground during the liberalism of the 60s and 70s, are out in th open again.

"Somehow, I feel that this type of racism has been brewing for some time, and is perpetuated by the climate of the country," said Elvira Williams of the Montgomery County NAACP. "The time is just right now for the doors to swing open."

Some observers, like Sonner, who is personally monitoring the status of five juveniles charged in a cross burning in Poolesville, say it is difficult to assess whether the number of incidents is really rising.

"I think we have had this type of behavior. Unfortunately, it is endemic to our society," Sonner said. "But it could be an increase in the number of reports of incidents, just as more women reported rapes after the importance of that was stressed."

Still, according to Mongomery Police Sgt. Richard Williams, hate group activity is a serious and growing problem. In Wheaton, for example, a young black woman returned home to find a note on her door that read: "You are being watched by the Knights of the Ku Klux Klan."

In Potomac last January, unknown persons made swastika patterns in the snow outside a Jewish home. The family had been victims of three previous incidents that started last Summer, police said.

"They're frightened and confused. One black family in Wheaton completely abandoned their living room after some of these incidents happened," Williams said. "You stay up all night after someone shoots BBs or throws rocks through your windows. It scares the hell out of people."

Montgomery was 94.5 percent white in 1970, according to statistics from the county's planning board. Since then, a slowly increasing number of blacks and other minorities have entered the county until the percentage of whites had dropped by 10 percent in 1980.

Police say there are patterns to the incidents. In Wheaton, whites are pitted against blacks; in Silver Spring, blacks against Hispanics. In Rockville and Potomac, Jews are more likely to be the victims, said Sgt. Williams.

There are new laws and new attempts to study the problem. It is now a felony to burn crosses, and state police now chronicle hate group incidents monthly, according to Lamont Edwards of the state police.

"There is a sense of anxiety that is nationwide and beginning to affect this county," said Montgomery NAACP President Roscoe Nix. "It's an illness that can be extremely dangerous to this country."