Hundreds of thousands of Nicaraguans voted today in well-organized elections that represented the first formal referendum on the ruling Sandinista Front since it seized power in a popular insurrection five years ago.

Mariano Fiallos, president of the Supreme Electoral Council, said at a news conference this evening that the Sandinistas appeared to have achieved their goal of an 80 percent turnout of the 1.55 million registered voters, although a definite figure would not be available until Monday. "We have surpassed 80 percent," Fiallos said.

There was no way to independently confirm the Sandinistas' estimate of the turnout. Long lines formed at many polling places in Managua even before the polls opened at 7 a.m. and again in the late afternoon after a midday lull.

State radio late tonight announced returns from 11 of the 3,892 polling places, and these initial results gave the Sandinistas 64 percent of the vote. Nationwide totals were scheduled to be released Monday morning.

The Independent Liberal Party, which tried to pull out of the race but was not allowed to do so, placed second with 13 percent in the initial returns. The Democratic Conservatives were third with 11 percent; they beat the Sandinistas in one polling site, in the town of Muy Muy in Matagalpa province in an area where U.S.-backed antigovernment rebels are active. The remaining votes from the 11 sites were divided among four other opposition parties.

Fiallos said that opposition party poll watchers were present at no more than 1,189 polling places, or less than one-third of the sites.

The elections are for a president, vice president and a 90-seat National Assembly. Sandinista presidential candidate Daniel Ortega already is chief of state in his position as coordinator of the governing three-man revolutionary junta.

Most voters who expressed an opinion to foreign journalists in Managua said they were voting for the Sandinistas, who are expected to win easily, in part because of a boycott by four opposition parties.

These voters praised the elections as the first free ballot in Nicaragua's history and said that they hoped the voting would be a blow to the Reagan administration by showing that the people support the Sandinistas.

But a sizable minority of those interviewed said that they would vote for one of the six small opposition parties participating in the race.

Several of these critics said that they feared that they would have trouble with authorities if they did not vote, and several said they wished that Arturo Cruz, the candidate of a boycotting coalition, had stayed in the election. He charged that conditions prevented a fair contest.

Some foreign observers expressed concern about the shortage of opposition poll watchers, although those poll watchers interviewed reported no irregularities in the voting. Officials of the small parties reported that they did not have the resources to station representatives at most voting sites. Polling officials are required by law to be impartial, but most are members or sympathizers of the Sandinista Front.

Fiallos reported only one confirmed violation of the cease-fire announced by the main body of antigovernment rebels. Rebels fired mortars at an electoral police vehicle near La Tronca in Matagalpa province and killed one policeman, he said.

Nine polling places near the Honduran border in Northern Zelaya province failed to open because of fear of a guerrilla attack, Fiallo said.

A Defense Ministry source said there was an unconfirmed report of a clash at 8:30 a.m. at the Costa Rican border post of Penas Blancas.

Army troops backed by tanks maintained a strong presence in the two northern provinces of Nueva Segovia and Jinotega, where the rebels normally are active, special correspondent John Lantigua reported. Troops were stationed in all of the six towns that he visited in the two provinces on the Honduran border, and officers said a rebel attack was feared but had not materialized.

Voters waited patiently at schools and other polling places until an "electoral policeman" signaled that they should enter the voting room. Each turned in a voting card that had been distributed during the four-day registration period in July, and the electoral junta of three officials checked a catalogue to confirm that the person was voting in the right place.

Each voter then received two ballots -- a blue one for the president and vice president, and a gray one for the National Assembly -- and withdrew behind a dark blue curtain to mark them. Ballots were deposited in plywood boxes, and the voter's thumb was dipped in bright red ink to prevent a second try.

The government mobilized 40,000 officials and police to run the polls. The police wore armbands marked "PE" for "electoral police," but many also wore shirts with round cloth badges saying "Sandinista police."

There was no way for officials to see how individuals had voted, because the ballots were not numbered. Voting was not compulsory, but officials marked both the catalogue and the voter card to show that a person had voted. Ortega has warned publicly that nonvoters "objectively" were helping the counterrevolution.

A middle-aged lawyer who voted for the opposition Independent Liberal Party in a well-to-do Managua neighborhood switched from Spanish to English to say that he had voted only because he feared reprisals if he abstained.

"You might have some troubles getting ration cards for milk for your kids if you don't vote," he said. He made the comment in English, he said, because "somebody probably is listening." His wife said she intended to spoil her ballot by marking the spaces for all of the parties.

Most voters who said that they were voting against the Sandinistas said that they had voted for the Independent Liberals, whose presidential candidate, Virgilio Godoy, tried to withdraw but was prevented from doing so by the Supreme Electoral Council.

But all those questioned predicted that the Sandinistas would win, and many expressed pride in the balloting. "These are the first free elections we've had in Nicaragua. I'm 65 years old, and this is the first time I've voted," said Concepcion Reyes. She said that elections under Anastasio Somoza, the dictator overthrown in the Sandinista revolution in 1979, were "always a farce."

Pro-Sandinista voters also were well aware that the elections were called in large part to impress public opinion in the United States and Western Europe. "These elections are a way of paralyzing the aggression of the United States," Cibeles Pineda, 35, said.

Special correspondent Lantigua added:

Voters trudged for miles down rural roads through mist-covered mountains early this morning to polling sites here in the north. Long lines moved slowly in Ocotal in Nueva Segovia province, and some voters complained about the wait.

In nearby San Fernando, however, a town where many residents have relatives fighting with the rebels, a group of farmers sitting on a porch said they were unsure whether they would vote.

"There is a war going on here, and there are not conditions for an election," one middle-aged farmer said. He and his friends said that they certainly would not vote for the Sandinistas if they decided to vote.

The Honduran-based Nicaraguan Democratic Force announced last week in Tegucigalpa that it would refrain from staging attacks on voting day until at least 5 p.m. Its radio station urged voters to spoil ballots by voting for more than one party.