Opponents of Loudoun County's extensive effort to slow home building filed more than 150 lawsuits against the county yesterday, launching one of the broadest legal challenges to a local government in Virginia history.

The suits, filed by builders, developers, longtime landowners and last-minute investors, seek to overturn all or parts of a far-reaching slow-growth law passed by Loudoun supervisors last month that would sharply reduce development in the nation's second-fastest growing county.

County officials, girding for the rush to Loudoun's historic, red-brick courthouse, announced this week that they plan to add $6 million to the legal defense fund they set up in 2000 after a slate of eight supervisors came to power on promises to stop sprawl.

The barrage of lawsuits, coming on top of dozens filed earlier, is tied to a wider political effort to push the slow-growth supervisors from office. Several board opponents said they hoped that elections in November will usher in pro-growth candidates who would overturn the new limits and make their suits moot.

"I'm sorry we're here today," said Jack Shockey, a large landowner, big-game hunter and former federal investigator who heads the pro-development group Citizens for Property Rights. Standing in the county courthouse in a zebra-skin jacket and his group's trademark red bandanna, Shockey said members of his group had no choice but to fight the county in court. "I'm sorry the Board of Supervisors didn't listen," he said.

The new rules slash the number of homes that can be built on a 300-square-mile swath of land between Dulles International Airport and the Blue Ridge Mountains. Backers noted that the county's population doubled in the 1990s, straining the environment, county finances and the patience of residents concerned about the impact on their quality of life.

But detractors have argued that the effort, which cut the total number of homes that can be built in Loudoun by more than 80,000, will harm not only landowners who hoped to subdivide their property but also families looking for affordable housing in the region.

County supervisors say they spent three years after their landslide 1999 election listening to slow-growth supporters and opponents alike. The carefully crafted and much-debated zoning ordinance that resulted will stand up to legal scrutiny, they said.

"This is about what we expected," said Supervisor James G. Burton (I-Mercer), who lives in the tiny community of Aldie in rural western Loudoun and is a vigorous slow-growth proponent. "That's why we took as long as we did -- to make sure that we followed the public process correctly and that we treated all aspects of the issue fairly and openly. I'm confident we will prevail."

The legal challenges began to trickle in in recent days , but the vast majority of the nearly 200 lawsuits were filed yesterday, which marked an important 30-day deadline for many types of suits.

Circuit Court Clerk Gary Clemens took the unusual step of setting up appointments yesterday with various teams of attorneys so they could submit their complaints without running into a major day-end traffic jam at the court counter. "It's like an assembly line," Clemens said.

With the zoning law they passed last month, the supervisors cut the number of homes that can be built throughout much of western Loudoun from one house per three acres to one house per 10, 20 or 50 acres, depending on the location and whether the homes are clustered to preserve open space. Supervisors also imposed wide-ranging environmental rules countywide that prevent construction near stream beds and other natural features deemed to be sensitive.

The lawsuits challenged those actions based on a host of legal notions. Many of the suits allege that county regulations amounted to an unconstitutional "taking" of their property because, they argued, cutting the number of houses allowed on their property makes it less valuable. Others allege that Loudoun's actions represented a "piecemeal" downzoning that unfairly -- and illegally -- treated similarly situated landowners differently.

Still other suits allege that the county's actions robbed them of "vested" rights to develop their property that they had locked in by investing in, and securing county approval for, subdivision plans.

The challenges to Loudoun's land-use regulations recall similar circumstances in Fairfax County more than a decade ago when the supervisors' efforts to slow development by downzoning thousands of acres were met with hundreds of court challenges.

Proponents and opponents of Loudoun's new law cite experiences in Fairfax to bolster their case. For instance, restrictions on building that could affect Fairfax's Occoquan reservoir were upheld. A more wide-ranging effort to slow growth in Fairfax by cutting commercial and industrial development was overturned by a Fairfax court, then appealed to the Virginia Supreme Court, which reversed the lower court.

An intervening election brought a new board chairman to power in Fairfax, and the board reversed many of the restrictions.

"Ultimately, the landowners got back much of what they lost," said one attorney who challenged the Fairfax rules and was in Loudoun yesterday to try again.

He had plenty of company. Michael Banzhaf, an attorney with Reed Smith, sent a form letter to firm clients warning that they "must file suit" by yesterday to protect their rights. Banzhaf said he filed 68 complaints. One common argument among many of them is that Loudoun's actions are exclusionary because they will result in sharply higher housing prices.

"If every jurisdiction decided that they wanted to have large-lot zoning, you'd have fewer opportunities to provide housing, and that would, of course, drive up the cost," Banzhaf said. "That's the downside of the 'smart-growth' movement."

A suit filed by attorney Francis A. McDermott on behalf of landowner Wray S. Dawson argued that county supervisors are "beholden to the interests of elitist Western Loudoun Interests" and their "carefully orchestrated and concerted campaign to curtail the property rights of other landowners within the county."

But Loudoun County Attorney John R. Roberts expressed confidence in his team of lawyers, many of whom helped defend Fairfax in similar lawsuits in the 1980s and early '90s.

He also dismissed as public relations spin arguments that the new law is elitist and rejected the claim that the new zoning will harm property rights.

Instead, he said, "it will have a long-term beneficial impact on property values" countywide.

Staff writer Maria Glod contributed to this report.