The ruling Sandinista Front claimed victory today in yesterday's nationwide elections as early returns gave it a 2-to-1 lead over all of the contesting opposition parties combined.

"The Sandinista Front is the big winner," President-elect Daniel Ortega, 38, told a news conference. "Nicaragua has won the battle of the elections, which is the most important battle."

The Sandinistas were taking 68 percent of the vote in races for both the presidency and the National Assembly, according to returns from 37 percent of polling places released this morning by the Supreme Electoral Council. According to these returns, the turnout was 83 percent, and 7 percent of ballots had been declared invalid because they were marked twice or otherwise spoiled.

A 68 percent Sandinista share of the vote would be lower than Sandinista leaders had predicted. They had said they would receive from 70 percent to 90 percent, and some western diplomats here had expected the front to surpass 70 percent.

Ortega said: "The Nicaraguan people have given their support to the Sandinista Front and at the same time to political pluralism, which the Sandinista revolution defends, by voting for other political forces."

State Department spokesman John Hughes said, "Regrettably, the Nicaraguan people were not allowed to participate in an election in any real sense of the word. The Sandinista electoral farce, without any meaningful political opposition, leaves the situation essentially unchanged."

The two top vote-getters of the opposition parties had come close to withdrawing from the elections in the two weeks before the vote. The electoral council rejected such a petition from the Independent Liberals, who were winning a 10.6 percent share.

A gang of youths sympathetic to the pro-election faction of the Democratic Conservatives' leadership disrupted the party's convention just before it was expected to vote to join the boycott. That party's early percentage was 12.4.

Three small opposition parties, which together had more than 1,000 observers at the polls, reported only a handful of minor irregularities. Foreign election observers -- including a Spanish government official, and politicians from Honduras and Britain -- also reported that voting was orderly and without anomalies.

"There were clearly no irregularities or any corruption in the electoral process yesterday," British Conservative member of Parliament David Ashby said. But, echoing recurrent criticisms by the U.S. government, Ashby also said that political pluralism had little meaning here because Nicaragua was "effectively a one-party state." This was disputed by a British Labor Party observer, Alfred Dubs, who called the election fair.

Officials of the three small political parties said in separate interviews that they believed the electoral council's estimate of both the vote count and the turnout. The abstention rate was important both as a sign of disillusionment with the Sandinistas and because the four-party Democratic Coordinator Alliance, considered the principal opposition group, had boycotted the election.

"As we see it, the abstention rate was low. It's perfectly reasonable that support for the Coordinator was in the order of 20 percent," Luis Guzman, a leader of the Popular Social Christian Party, said. His party sent out 360 poll-watchers yesterday and thus had the second-highest number of opposition observers, after the Marxist Nicaraguan Socialist Party.

Some leaders of the Coordinator suggested that the Sandinistas somehow had manipulated figures to inflate the turnout, but they admitted that they had no proof. Former junta member Arturo Cruz, who would have been the Coordinator's candidate if he had run, said the size of the turnout was "irrelevant" compared to the issue of whether Ortega as president would try to reach an understanding with the opposition.

"If the country continues to be governed from the inner sanctum of one party . . . then it will be very serious," said Cruz, who flew to Managua from his home in Washington on the night before the elections. His group charged that minimum conditions for a fair vote, such as press freedom, did not exist.

Ortega, who already is chief of state as coordinator of the governing three-man junta, is to take office as president on Jan. 10. He expressed satisfaction that voters had supported the Sandinistas despite economic hardship and U.S. "aggression." He, his vice president Sergio Ramirez and the 90 deputies in the National Assembly are to serve six years. The returns indicated that the Sandinistas would enjoy a comfortable majority in the assembly, or legislature, that is to write a new constitution.