Six environmental groups went to federal court yeserday in an attempt to halt construction of an $850 million oil refinery in Portsmouth, Va., but the action came too late to stop bulldozers, which already had began clearing the site.

When attorneys for the National Wildlife Federation and five other groups discovered yesterday that the bulldozers had begun work Monday, they said they would have to rethink their legal strategy and possibly seek quicker action.

Wildlife Federation attorney Thomas G. Tomasello said it would be at least a month before the U.S. District Court for the District would respond to the environmental coalition's call for a permanent injunction. By that time, the 624-acre site for the huge refinery, which would be the largest on the East Coast, would probably be cleared. The refinery is to be built where the James River runs into the Chesapeake Bay.

The Washington-based company building the refinery, Hampton Roads Energy Co., is determined to move ahead.

"Their [the environmental groups'] objective is delay, delay, delay," said company Vice President Robert E. Porterfield. "They're hoping we'll lose money and go away. But we're staying."

In their suit, the environmental coalition said there is a "50 percent chance of a catastrophic [oil] spill during the projected 25-year life of the refinery." The refinery would have a capacity of 175,000 barrels a day.

The National Wildlife Federation, in a statement accompanying the suit, said "a serious oil spill in the Chesapeake Bay could be devastating, not only to wildlife species and the Chesapeake's fishing industry but to the huge outdoor recreation industry as well."

Construction of the refinery is expected to bring about 3,000 temporary jobs to aging Portsmouth, whose industrial growth has been stunted because the city has not been able to expand beyond its cramped borders.