Half a dozen Greenpeace environmental activists gathered here under a moonless sky near the Bethlehem Steel plant at 5 a.m. today and collected fetid, wet sludge from an industrial landfill while two security policemen peacefully stood by and watched.

The activists were trespassing on Gray's Landfill, owned by Bethlehem, in preparation for more trespassing and civil disobedience later in the morning to protest what they call illegal toxic waste dumping by the steelmaking giant. Greenpeace members were planning to be arrested. The question was, what were the police going to do?

"Nothing," the taller of the two lawmen told a member of Greenpeace. "As a matter of fact, if you'd be kind enough to move your vehicle, we'll leave."

It was not the reaction Greenpeace expected, coordinator Helen Perivier said. Nor did the activists expect to be allowed onto the steel mill's entrance on Riverside Drive several miles away an hour later to dump six 55-gallon drums of the sludge in the road and handcuff themselves to their truck, disrupting the morning shift change.

"They're ignoring us," said Perivier. "But what they don't understand is you can't ignore us, and we won't go away."

Traffic entering the Sparrows Point plant in the Baltimore suburbs backed up more than a mile and was being redirected to other plant entrances. At about 6:30 a Bethlehem guard advised the protesters cuffed to the truck, "You are trespassing, blocking traffic and dumping," and asked them to leave.

"Well, sir," replied 25-year-old Marco Kaltofen of Boston, whose plastic safety coveralls were covered with greasy, black slime from the sludge-gathering operation, "you are dumping chemicals into the Chesapeake Bay, and we are telling you to stop."

The confrontation made for good street theater on the night-dark banks of the Patapsco River, but was quickly over when the guards cleared the area of everyone but the six protesters, who stayed all day, finally leaving of their own volition about 6 p.m. Bethlehem employes worked this afternoon to clean up the sludge.

Greenpeace, an international organization claiming 9,000 members in Maryland, maintains that Gray's Landfill is an "unlined, unlicensed toxic waste dump" that is leaching such hazardous chemicals as benzene, toluene, chloroethane, acetone, trichloroethane and chloroform into groundwater and into the Patapsco River, which feeds the Chesapeake.

Bethlehem spokesmen refused comment, other than to say that the Greenpeace action made hundreds of workers late and plant production was interrupted or delayed. But state Health Department spokesman Linda Smeyne conceded that chemicals have been found in the groundwater below Gray's and that it is "entirely possible" that toxins are leaching into adjoining Bear Creek and on to the Patapsco.

Greenpeace leaders say Bethlehem enjoys special consideration from the state because of its size and the precarious position of the American steel industry. But Smeyne denied that. "We try to deal with all industries in an evenhanded way," she said.

Smeyne said Gray's Landfill operates as a nontoxic landfill under a 1982 permit, but said evidence indicates there may be toxins and the state is working with Bethlehem on a plan to reassess the toxicity of sludge dumped there. "The nonhazardous status may or may not be changed," as a result, she said.

The sludge -- 2,200 tons a month, according to Greenpeace -- comes from Bethlehem's Humphrey Creek wastewater treatment plant, where byproducts from the steel plant and five other Baltimore heavy industries are handled. "This stuff is chemical soup," said Kaltofen, a chemist, as he shoveled the slime, "and we feel this landfill has to be closed."