For two decades, the Great Western Gun Show has been the nation's largest legal marketplace for weapons and ammunition, drawing many thousands of shoppers to the sprawling Los Angeles County fairgrounds four weekends a year. But its long and profitable run there is about to end.

In another sign of rising alarm over gun violence, the Los Angeles County Board of Supervisors today banned the sale of weapons and ammunition from taking place on any county-owned property, a move intended to evict the Great Western Gun Show from the fairgrounds and force others like it to relocate.

The unusual ban is the latest in a growing wave of new restrictions on weapons that elected officials across California are endorsing in what has become a long summer of discontent for gun merchants and buyers in the state as well as powerful interest groups such as the National Rifle Association.

In just the past week, California lawmakers have approved legislation banning the manufacture or sale of unsafe handguns in the state, so-called Saturday night specials, and subjecting other ones to strict new consumer safety standards that would ensure no misfires. Last month, Gov. Gray Davis (D) also signed into a law a measure giving the Golden State the nation's most far-reaching ban on assault weapons.

The crackdown against gun shows in Los Angeles County comes just two weeks after white supremacist Buford O. Furrow's alleged shooting rampage with a submachine gun in the city. It left three young children, a teenage counselor and a receptionist at a Jewish community center's summer camp wounded. Furrow subsequently used a handgun to kill a U.S. postal worker who was targeted because he was a minority, authorities said.

County supervisors said they no longer want to allow the gun shows to use the fairgrounds or other county-owned sites here, such as armories or convention centers, because they believe the expositions have become a hub for sophisticated illegal weapons buying and trading that is too large and difficult to police. At the last Great Western Gun Show here this summer, federal agents working undercover said that they discovered an assortment of subtle and clever criminal activity among merchants and buyers alike.

"We have too many innocent children who are becoming victims of weapons that in some small way we have been helping to sell," said Supervisor Yvonne Braithwaite Burke, a sponsor of the ordinance.

"This is but a small step, but it's a very important step," said Supervisor Gloria Molina. "We have been finding violations of gun laws on our own property."

The ordinance passed by a 3 to 2 vote among county supervisors. It will take effect in about six weeks and could cost the county several hundred thousand dollars in annual revenue. But a majority of supervisors said that they were willing to lose the money.

"We should not be in the arms business," said Joel Bellman, a spokesman for Supervisor Zev Yaroslavsky, who led the campaign for the measure.

Organizers of gun shows aggressively fought the ban and are vowing a court fight to have it overturned. Some say that it will have little effect because they will move the shows to other parts of Southern California. Just this past weekend, for example, the Crossroads of the West gun show was held on public property in neighboring Orange County. But other gun show managers sound worried because finding large private venues in the region could prove difficult. And there may be more obstacles ahead for them.

State lawmakers are not allowing new gun shows to receive permits on California property until they resolve whether to impose tougher restrictions. Several such bills are now being debated.

"Firearms are not the problem," said Chad Seger, the manager of the Great Western Gun Show, which on some weekends draws nearly 100,000 customers to the county fairgrounds. "The problem is that we are not prosecuting criminals, and we are letting illegal guns on the street. But that is not a gun show issue."

Like many of its counterparts, the Great Western Gun Show is a huge mart featuring both antique and state-of-the-art weaponry, as well as ammunition, body armor and survivalist gear, all for sale.

Gun shows across the nation have come under increasing scrutiny in the aftermath of the massacre this past spring at Columbine High School in Littleton, Colo., in which two students firing assault weapons on the campus killed 12 of their classmates and one teacher. A weapon used in the attack had been purchased at a gun show by an 18-year-old girlfriend of one of the teenage assailants. In Congress and in some state legislatures, there are growing calls to intensify background checks of weapons purchasers at gun shows and to forbid anyone under 21 from buying items there.

The drive to oust gun shows from county property here gained momentum recently when both Los Angeles County Sheriff Lee Baca, a former member of the National Rifle Association, and Los Angeles Police Chief Bernard C. Parks announced strong support for the measure. Gun control groups in California also have been lobbying for it and plan to begin pressing other counties in the state to take the same step.

"The shows have become a marketplace for illegal activity, or at least a magnet for people who want to engage in illegal activity," said Luis Tolley, the western regional director for Handgun Control.

In one recent case that inspired the ban, federal agents arrested a merchant at the Great Western Gun Show and accused him of selling weapons parts illegally. Law enforcement officials said the merchant first sold undercover officers a kit to assemble most of the parts needed for a particular machine gun. That transaction was legal. But officials said the man then told the undercover buyers he would sell them the remaining crucial part to complete the machine gun at another date and location, and later met them there.

"We should not be letting public space get used to foster this at a time when we're taking so many other steps to restrict Saturday night specials and assault weapons," Bellman said. "It's complete hypocrisy."