President Jose Napoleon Duarte's Christian Democratic Party claimed a surprise victory in today's legislative elections based on initial, unofficial returns and on a television exit poll that has been reliable in the past.

The president's conservative rivals declined to comment, saying that they had not yet seen initial returns.

If the Christian Democrats' assertion of victory proved true, it would represent a major upset and would bolster Duarte's policies and his efforts to negotiate an end to this nation's five-year-old civil war.

Projections were not yet available in nationwide municipal elections, which also were held today. Unofficial returns based on reports by party poll watchers are expected Monday for both the legislative and municipal races, while official results are anticipated later in the week.

"We have at least 31 seats . . . based on the information that we have," Duarte said this evening. That would give the Christian Democrats a majority in the 60-seat Legislative Assembly, where the conservatives currently hold 34 seats. Most independent observers had forecast that the conservatives would retain control of the legislature.

Duarte based his estimates on the exit poll and unofficial returns from 12 percent of polling places. He said the Christian Democrats were winning 55 percent of the vote nationwide versus 42 percent for a conservative coalition that is their principal opponent.

Christian Democratic Party Secretary General Jose Morales Ehrlich, who is a candidate for mayor of San Salvador, also claimed victory, based on the television exit poll and on opinion polls taken before the election.

The Miami-based Spanish International Network projected that the Christian Democrats were sure of winning 32 seats in the assembly and probably would win one additional seat. Their ally in the assembly, the small Democratic Action party, was projected to win one seat, the network said.

A coalition of the nation's two largest conservative parties -- the Nationalist Republican Alliance and the National Conciliation Party -- was projected to win at least 22 seats and probably 25, it said. The remaining seat was projected to go to one of five small other parties.

Correspondent Pedro Sevcec said that the network questioned more than 11,000 voters as they left the polls. The network correctly projected results of last year's presidential runoff election to within 1 percent, although projections were more difficult this year because individual forecasts were necessary for the outcome of assembly races in each of the nation's 14 provinces.

Conservative leaders criticized the exit poll today, saying that it was not authorized by the government and interfered with the voting. The network responded that it needed no authorization and that participation was voluntary.

The voting was marked by moderate organizational problems and significantly less violence than occurred during past elections.

By late this afternoon, the armed forces had reported only eight incidents of harassment of troops and civilians by El Salvador's left-wing guerrillas. The insurgents are boycotting this election as they did the ones in 1982 and 1984, but they refrained from attacking cities or towns as they did in those years.

"On a normal day, there is more terrorist activity than there was today," armed forces spokesman Maj. Carlos Aviles said. Only one person was reported killed, a seminary student who was killed by machine-gun fire after his vehicle was stopped by insurgents in eastern San Miguel province, Aviles said.

The relative lack of violence appeared to result from the armed forces' improved performance and from a decision by the guerrillas not to disrupt the voting on a large scale. Guillermo Ungo, the exiled leader of the rebel alliance's civilian wing, said in a telephone interview that the left had agreed that it "wasn't going to attack the electoral process militarily."

In an unusual public criticism of his guerrilla allies, Ungo also expressed disagreement with the insurgents' recent series of raids on town halls in the countryside. The left's civilian branch normally tries to present a united front with the military wing despite known differences between them.

"It could be a lack of comprehension of political phenomena on the part of certain elements" in the military wing, Ungo said of the burnings of more than 20 town halls in recent weeks. "It doesn't seem to me that it has been the best way."

Voting was not carried out in approximately 10 percent of the nation's 262 municipalities because they were in guerrilla-dominated territory, according to Salvadoran radio reports.

Turnout was light in San Salvador and several other cities when the polls opened at 7 a.m., but it gradually increased during the day. Many voters went first to Palm Sunday services and then to the polls, and some waited in line to vote holding palms and simple wooden crosses.

Some political leaders and polling officials said that turnout appeared lower than in last year's two rounds of presidential elections. They noted that there was less interest in legislative and municipal elections and that several changes in the electoral laws this year tended to discourage voting.

In particular, voters were required today to cast their ballots in the towns or provinces where they were registered. Last year, special polling sites were established nationwide for the several hundred thousand persons who have moved far from their hometowns because of the civil war. These polling places were dropped this year because they were considered to have contributed to widespread organizational problems in the first round a year ago.

Despite the change, some organizational difficulties plagued voters today. Voting began three hours late in two large suburbs of San Salvador because the Central Elections Council delivered the wrong voters' lists. That problem was solved, but poll watchers at a polling place in one of the suburbs, Soyapango, reported that 60 percent of voters were turned away because their names did not appear on the lists