About 200 opponents of nuclear development held an emotional memorial service in Lafayette Park yesterday for Karen Silkwood, the Oklahoma plutonium worker who died four years ago in a much-disputed car crash.

Speakers vowed to win their long-term war against the onslaught of nuclear power and to win a $2.5 million civil suit filed against the Kerr-McGee Nuclear Corp., where Silkwood worked, and the Federal Bureau of Investigation.

"The conditions at the Kerr-McGee plant were outrageous," charged Sarah Nelson, co-chairperson of the National Organization for Women's labor task force.

"If we proceed with nuclear power as it is now designed, we're asking for a cancer epidemic such as we have never imagined," said Nelson.

But "we faced these same forces in the 1960s" she continued, calling up memories of the antiwar movement, "and we won . . . So we know it can be done."

Dan Sheehan, the Silkwood family attorney, predicted that "we'll all be standing here within a year and talking about how individual people . . . have brought one of the major industries in the United States (and) the FBI to the bar of justice."

The Oklahoma State Police, the FBI and a congressional subcommittee have all officially concluded that Karen Silkwood's death was accidental, but few nuclear safety activists have been willing to see it that way.

With hearings, articles, lawsuits and rallies like yesterday's, they have raised and reraised questions about safety practices at Kerr-McGee, about the mysterious plutonium contamination of Silkwood's home, about the disappearance of documents she supposedly was carrying with her when she died, and about the death itself.

Silkwood was driving to an appointment with a union official and a newspaper reporter when her auto veered off the highway and into a concrete wall. She died instantly.

An autopsy revealed traces of alcohol and methaqualone, a sedative, in her bloodstream.

About $100,000 has been sent so far in a cross-country investigation of nuclear plant practices and alleged government and private surveillance of the antinuclear movement, according to Sheehan.

Sheehan said yesterday he could not explain how Silkwood and her home came to be contaminated with plutonium in the weeks before her death. But he said he would establish that Kerr-McGee was legally responsible for the contamination.

Kerr-McGee has denied any negligence. A report issued in 1975 by the U.S. Atomic Energy Commission concluded that Silkwood had probably exposed herself to the contaminant.

The portion of the Silkwood family lawsuit that deals with the contamination and Kerr-McGee's alleged responsibility for it is set to be heard by a U.S. judge early next year. The other part of the suit, focusing on alleged violations of Silkwood's civil rights by Kerr-McGee and the FBI, has been held up pending an appeals court ruling on technical and jurisdictional questions.

Yesterday's rally commenced with a five-mile run around the White House and the Mall - an apparent effort to heighten interest and attendance. The top 50 finishers received antinuclear T-shirts as prizes.

Despite repeated entreaties from the podium, few of the runners remained for the oratory and music that followed.

Besides Nelson's and Sheehan's addresses, the agenda included an original pep song by Peter Jones of Bethesda in which he had President Carter proclaiming:

"And just to prove that nothing can go wrong. I'm going to build one (a nuclear plant) on the White House lawn."