In 1976, the last time Mayor Coleman A. Young ordered a tough curfew on city teen-agers, he bragged that he had the "baddest gang in town"--the Detroit police--to enforce it and challenged youths who didn't believe him to "get it on" and find out.

In a month, 1,090 boys and 153 girls under 18 years old were arrested for curfew violations. On the "mean streets" of the 5th police precinct on the city's east side, police used the curfew to help break up activities of the B.K.s, the Flynns and other menacing gangs.

Asked last Saturday night where those gang members are today, a desk sergeant said, "They'd be in one of two places: Jackson state penitentiary or dead."

Now, after a first-quarter crime-rate jump of 14 percent and the killing of two mothers and an 8-year-old boy during one three-day period early last month, Young again has ordered everyone under 18 off the streets by 10 on weeknights and 11 p.m. on weekends.

This time, his promise to lawbreakers was a stern "if you mess up, we will nail you." Curfew violators' parents will be summoned to pick up their children. Violators may have to spend a night in jail or face misdemeanor penalties of up to 90 days in jail and/or fines as high as $500.

The local chapter of the American Civil Liberties Union is threatening to seek court action. ACLU Executive Director Howard Simon said that courts have held previous curfews to be "an infringement on freedom of movement and travel and on the rights of parents to direct the upbringing of their children."

But many merchants, editorial writers and judges contend that public sentiment is on Young's side.

According to police, the toughest street kids here are drug couriers, many working for a network of drug dealers who use teen-agers and younger children as runners and holders.

Some police officials contend that young dealers are now fighting each other over drug-dealing turf and that their battles are responsible for much of the increase in street crime.

But, police report, the city also has children who bust into cars, burglarize homes, snatch purses and hold people up, often to get money to spend in a host of video game arcades that stay open very late.

Gold chains are also a popular item. Some youngsters ride through open downtown plazas on bikes snatching chains off the necks of strollers.

Early Saturday morning, police Sgt. John Wing and another officer were arresting a drunk in the Greek area of the city when an 18-year-old ran past, ripped two chains from the neck of the man being arrested and darted into the crowd. The teen-ager was apprehended a few minutes later.

"How much nerve does it take to steal a gold chain off a guy with a cop hanging on his arm?" Wing asked.

Between 1979 and 1981, Detroit, once considered the murder capital of the nation, reduced its crime rate by 30 percent, before witnessing a 5 percent increase in 1982 and a 14 percent boost during the first quarter of this year.

During the first four months of 1983, police arrested 7,204 persons for violent crimes. Of that number, 1,452 were under 18.

The Wayne County jail is reportedly full. County juvenile court cases are up 13 percent. Admissions to the county youth home have risen 10 percent. And the number of cases on file in the city's trial court is at its highest level since the 1967 riots.

"We are appalled at the number of gun cases," said Record's Court Chief Judge Samuel Gardner after the mayor announced Wednesday that the curfew would begin Friday night. "It seems that almost every young punk on the streets of Detroit has a gun."

Yet, the other noteworthy statistic is unemployment, which stands at 19.7 percent for the city as a whole--nearly twice the national average--and 48.6 percent for youths aged 15 to 19.

Among blacks, who make up two-thirds of the city's population, 32.7 percent of persons all ages who are looking for work can not find any. Among black youths, the unemployment rate is 66.7 percent.

The mayor says that he has sympathy for the unemployed. But, he added, "No amount of hardship can justify a mugging or assault or the promiscuous use of firearms."

Prowling the streets of the east side, Patrolmen Joanna Graves and Mike Sheehan stopped several clusters of youths. There were four girls returning from a fast food restaurant, including one who said she was 20 but had no identification, and two 15-year-olds.

About 12:30 a.m., they stopped a 10-year-old who was standing outside the entrance to a store with a Doberman pinscher and waiting for a 13-year-old to come out.

But warnings were the order of the night. No arrests were made; no citations were handed out.

Although many of the youths now affected by the curfew were in grade school during the last one, many have perfected evasive techniques. Hardly anyone carries identification; most volunteer ages above 18 and are tripped up only when their math falters after they are next asked their date of birth.

Others are rescued by adult bystanders, who become instant guardians saying, "He's cool; he's with me."

"These kids are going to feel like they're being harassed," Graves said. "It's going to be pretty bad. After four or five times, these kids are going to jail."