Polling stations have closed in Nicaragua for presidential elections. Election workers were expected to issue preliminary results later in the evening, and report a final tally on Monday. Brian Wagner reports from Managua.

Polls began closing across Nicaragua as scheduled at six o'clock local time. Some voting stations in Managua continued to operate, however, after residents complained that some were still waiting to cast their ballots. Long lines formed at some stations throughout the day because of delays caused by technical problems.

Several minutes after the scheduled closing time, officials locked the gates at one polling station located at a school in downtown Managua. One election monitor for the Sandinista party, Jose Antonio Martinez Valle, said a few voters waiting in line were allowed to remain to cast their ballots. He adds that monitors from three other parties were stationed at the school during the day to guard against problems.

Valle says voting was peaceful, and no voters were harassed or insulted by members of opposing parties. He adds that monitors had been instructed on how to behave during the vote.

Some 17,000 domestic election observers were deployed across the Central American nation, as well as 1,000 monitors from abroad. The Carter Center of former U.S. President Jimmy Carter sent 60 monitors, including the former president himself.

The chief of the center's mission in Nicaragua, Jaime Aparicio, said he received reports from monitors in several cities throughout the day.

He says improper ballots were delivered to polling stations in some cities, due to printing problems. Other stations opened late because of delays in the accreditation of election monitors. But he says, in general, the voting process was under control.

Aparicio says the relatively high number of observers has discouraged possible fraud. He says observation missions help to prevent a massive fraud in the presidential vote. But he says minor problems may take place in certain areas, which could affect the outcome of legislative elections.

One area where the Carter Center already has been effective is in pressuring the Nicaraguan government to respond to allegations of voter manipulation ahead of the vote. Aparicio says the group spoke out after critics accused some parties of seeking to delay the distribution of identification cards in an effort to suppress opposition votes.

The Carter Center and other foreign observation missions are expected to report Monday on their findings about the vote.

Opinion polls showed former Sandinista President Daniel Ortega in in the lead going into Sunday's vote. To win, a candidate needs 35 percent of the vote and a five-point lead in the first round, otherwise a run-off election will be held next month.