LAS Palmas park in this valley community north of downtown Los Angeles looks like any other urban recreation area: picnic tables, basketball courts, tire swings, a softball field with lights.But after a rash of gang violence this summer, the chatter on the courts subsided and the crack of bats on the diamond dissipated. So town officials passed an emergency ordinance banning gang members from the park. The result has been a return of children to the swings - but also a lawsuit from the American Civil Liberties Union (ACLU), which believes the statute tramples some basic constitutional rights. The legal fracas over the park raises questions about how far the arm of the law should extend in trying to stem gang violence that has reached unprecedented levels in southern California. Politicians and law enforcement agencies are always looking for new tools to keep some of Los Angeles County's estimated 100,000 gang members from committing their mayhem. But civil libertarians balk when they believe the laws are so broad and vague that they single out classes of people or flout the Constitution in trying to restore order. The ACLU won one battle earlier this month when a state court struck down a first-of-its-kind measure allowing authorities to hold parents responsible for the gang activities of their children. The San Fernando ordinance will provide one more test of the limits of the crackdown on crime, and one that will be watched in more than just a few lawyers' offices. For even as the ACLU was challenging the ban here, Pomona, another L. A. suburb beset by rising gang violence, was passing a similar ordinance, and the Los Angeles City Council is considering a sweeping anti-gang measure of its own. "It is something that can at least be done to give parks back to the people who use them," says Sandi Gibbons of the Los Angeles County District Attorney's office, which drafted the San Fernando law. "It shouldn't take an act of courage to take your child to a park." The mere consideration of such laws underscores the growing frustration over street gangs in the nation's second-largest city. By year's end, police expect the number of gang-related homicides in Los Angeles County to approach 750 - a record for the fifth year in a row. In 1984, gang murders accounted for 15 percent of the overall homicides in the county. Today they account for 35 percent. Those who deal with the problem every day have no easy explanations for the surge. Police and social workers cite the usual suspects: drugs, greater firepower on the streets, society's desensitization to violence, a growing underclass. Much of it is old-fashioned rivalry and retaliation. "It is partly the growing hatred of one gang against another," says Sheriff's Sgt. Wes McBride, a 25-year veteran of the force. "The vast majority of the motives in murders tend to be rivalries." Gang membership keeps mushrooming. Police estimate there are now 950 cliques county-wide representing 100,000 members, double what it was five years ago. Almost everyone agrees that more programs aimed at the social and economic problems underlying gang activity are needed. Almost everyone also agrees vigilant law enforcement is needed. The question is how much and what kind. The San Fernando ordinance was passed after a series of gang incidents in the park, culminating in a shoot-out that left an innocent mother and her three children wounded. It imposes up to a $250 fine on gang members, identified in a list compiled by police, who enter Las Palmas. The emergency order is aimed mainly at members of two rival Latino gangs, the Shakin' Cats and SanFers. The ACLU charges that banning one group from the park, in addition to violating the constitution, will lead to abuses: Someone could be cited just because of his or her ethnic origin. "You could have a tattoo and be kicked out," says ACLU attorney Robert Malley. The group says the ordinance has pushed gang violence elsewhere in the neighborhood. It suggests controlling the problem by setting up a police substation near the park, fixing broken lights, and pursuing programs that give youths alternatives to joining gangs. Police say they don't have the resources for a permanent presence in the area. They agree the gang problem has been dispersed, but believe it is now more controllable, less threatening, and has restored order to the park. Residents concur. "It has worked terrifically," says San Fernando Police Chief Dominick Rivetti. The law being considered for Los Angeles would not make parks off-limits to gang members. It would, instead, make it a crime for a gang member to assemble in any park, beach, or recreation area with the "intent" to commit a crime, a difference the Los Angeles City Attorney's office, which drafted the proposal, believes will make it challenge-proof. The ACLU says the measure may be on stronger legal footing than the San Fernando law, but sees it as just another "Band-Aid" solution to the gang problem.