More than 100 people jammed the meeting chamber of the Fairfax County Board of Supervisors last night to protest plans for a huge energy-generating incinerator just off Shirley Highway near the Lorton Reformatory.

The board, which must decide whether to approve a 23-acre site for the trash-burning facility at the regional landfill at Lorton, is scheduled to decide the issue next Monday.

The proposed $268 million plant, which would be one of the largest such facilities in the country, is designed to extend the life of the landfill, which county officials predict will be full by the end of the century.

Opposition to the plant, which seems to be widespread in the Lorton community and throughout southern Fairfax County, focuses on the potential impact the facility would have on the environment.

"If your decision is to build," said Marcia Hanson, head of the Federation of Lorton communities, "you will have dashed our hopes for a clean, safe and healthy environment."

Supervisor T. Farrell Egge, a Republican who represents the area and who has sided with many of his constituents in opposing the facility, said: "For years and years the people in the Lorton area have been treated as second-class citizens who shoulder the responsibilities for the rest of Fairfax County. I want to make it clear tonight that I find such an attitude totally unacceptable."

Egge and many of the speakers before the board emphasized the potential hazards posed by dioxins, a class of highly toxic, cancer-causing chemicals that are emitted from trash-burning facilties of the type proposed for Lorton.

"Can existing pollution control technology eliminate the risk from dioxin?" asked Philip Chabot, a community activist who opposes the plant, in a written statement. He answered his own question: "No."

A consultant's environmental study for the county concedes that the plant will spew out air pollutants, but says that the levels will be within accepted safety standards.

As an alternative to the plant, Egge has proposed that Fairfax build smaller facilties around the county. Other supervisors, apparently leery that one of the plants would land in their districts, have not embraced the idea.

Responding to one speaker, Supervisor Nancy K. Falck (R-Dranesville), voiced a frustration that other county politicians share privately. "You face two choices" with trash, she said. "You either bury it or you burn it, and we've run out of space to bury it."

Her comment was met with cries of "No! No!" from the audience, a number of whom advocate a mandated trash screening and recycling program.