What is believed to be one of the most expensive garbage dumps in the nation finally opened yesterday morning near Laytonsville in upper Montgomery County, capping a decade-long controversy over where to put the new landfill and how best to make it safe.

About 35 protesters were on hand as the first garbage trucks came rumbling down to Rte. 108 and Riggs Road to dump the county's refuse into the lavish site, referred to sarcastically by some county officials as the "Taj Mahal" of landfills for all of the costly studies and expensive trappings. Some of the protesters made speeches, while a few cried openly, lamenting the end of their four-year battle to prevent the 550-acre garbage dump from opening.

For more than three years, the landfill issue has politicized Laytonsville's entire community. With their yellow T-shirts and pasteboard posters, they have packed the weekly meetings of the County Council. They have threatened to run their own political candidates against council members who supported funding the landfill. And Monday night, the emotional eve of the landfill's opening, the residents who had lost their fight in court and in the council chambers made one last appeal to the heavens during a tear-filled prayer session by asking God to block the garbage trucks.

"We thought we'd tried everything over four years," said Laytonsville Mayor Charles T. White. "We thought we'd bring it to an end with prayer and thought that that might help."

The Laytonsville residents have argued that fissures in the ground beneath the landfill could led to contamination of the community's drinking water. Also, they said that the landfill violates a 1978 ballot referendum--approved by 70 percent of county that prohibits spending county funds to operate a landfill in a residentially zoned area. The legality of that measure is still being decided in court.

At the end of the day, a county circuit court judge dashed the citizens' last hope when he denied their request for a temporary injunction blocking the landfill's opening. Judge Philip M. Fairbanks, in a nine-page ruling, said the public interest would best be served by allowing the landfill to operate as scheduled while remaining legal issues are battled out in the courts.

"Any injury which may result is neither immediate nor irreparable," Fairbanks wrote in his opinion. He said he was ruling to let the landfill stay open after "balancing the immediate necessity for trash disposal in a county of more than 600,000 inhabitants as opposed to the possible injury to a relatively small group of citizens."

The Laytonsville dump replaced the overburdened garbage dump on Gude Drive in Rockville, which was closed under court order last Friday. The new landfill, which will handle about 1,400 tons of garbage each day, actually takes up only 160 acres in the middle of a 550-acre site, barely visible from the road and surrounded by expensive landscaping.

The total price tag on the new dump, including a garbage transfer station, is more than $40 million, according to Alexander J. Greene, a top aide to County Executive Charles W. Gilchrist. That cost will have to be recovered through higher collection fees.

"The landfill is open and taking refuse," Greene said. "It works well."