A U.S. organization that has received substantial funds from the National Endowment for Democracy, a federally financed agency, has been funneling most of the money to opponents of the Sandinista government in Nicaragua, an official of the group said yesterday.

The organization, called Friends of the Democratic Center in Central America (Prodemca), has used the government money to help the opposition newspaper La Prensa and an anti-Sandinista human rights group.

Prodemca also has placed full-page advertisements in The Washington Post, The New York Times and The Washington Times urging Congress to support President Reagan's request for $100 million in military and nonlethal aid to the Nicaraguan counterrevolutionaries, or contras, but an official of the group said the ads were not purchased with government money.

"We support military assistance to the Nicaraguans fighting for democracy," said the ad.

Over the last year and a half, Prodemca has received about $400,000 in grants from the National Endowment for Democracy (NED), Penn Kemble, a member of Prodemca's executive committee, said yesterday.

"The money . . . is pretty much a pass-through," said Kemble, adding that none of it "goes for the ads." He said Prodemca took a certain percentage of each grant for an "administrative set-aside . . . . One could consider that part of our budget."

Rep. Daniel A. Mica (D-Fla.), chairman of the international operations subcommittee of the House Foreign Affairs Committee, which oversees the NED, said he was "mighty upset" about the newspaper ads.

"I want a full report immediately. If they spent one cent of government money on the ads , we'll throw the book at them . . . . I've had it with NED and the grantee if this is what's going on. It shows extremely poor judgment, terrible judgment, to get involved in a partisan issue."

Mica voted for the contra aid bill in committee March 6.

"NED just seems to go from one blunder to another," said Rep. Hank Brown (R-Colo.), a subcommittee member and a frequent NED critic.

The NED was created by the Reagan administration in 1983 to campaign for democracy and against communism. Last year, it became embroiled in a controversy over interference in French politics when the Paris newspaper Liberation disclosed that the Free Trade Union Institute, established under NED's aegis, gave money to an anticommunist group with murky ties to a far-right paramilitary organization.

NED director Carl Gershman is a neoconservative and former senior aide to former U.N. ambassador Jeane J. Kirkpatrick. The Free Trade Union Institute director is Eugenia Kemble, sister of Penn Kemble.

Penn Kemble serves on the board of the Democratic Institute, another group funded by the NED. "Ask our mother if you think this is a conspiracy," he said. He is also chairman of the executive committee of the Coalition for a Democratic Majority, a small neoconservative group within the Democratic Party.

Prodemca was founded in 1981 with grants from right-wing foundations -- the Smith-Richardson Foundation, the Olin Foundation and the Carthage Foundation. Many of the signers of its ad are neoconservatives, but some were prominent Carter administration officials: Zbigniew Brzezinski, national security affairs adviser; David Aaron, Brzezinski's deputy; and Richard Holbrooke, assistant secretary of state.

"Prodemca appears to be flying under false colors in portraying itself as a citizens' organization when it is actually a vehicle for distributing funds to anti-Sandinista organizations," said Aryeh Neier, vice chairman of Americas Watch, an independent human rights group that opposes contra aid.

"Nothing that we've done with funds we've received from the endowment has been used to influence U.S. policy," Kemble said.

Brown recently received a Prodemca letter lobbying him to support contra aid, a position he holds.