ABIDJAN, Ivory Coast (AP) _ President Felix Houphouet-Boigny appeared bound for overwhelming victory today in the country's first contested presidential election, which the opposition said was stolen through massive fraud.

It was the first time a leader of one of Africa's one-party states was being challenged at the polls since a wave of protests and rioting for democracy began sweeping sub-Saharan Africa early this year.

With about one-third of the ballots counted by midday today, Interior Ministry official Claude Regnier said Houphouet-Boigny had captured about 85 percent of votes. The count was being done at the ministry.

But the 85-year-old president's sole challenger - socialist-minded history Professor Laurent Gbagbo - said a computer count by his Popular Front gave Gbagbo a lead of 51 percent to Houphouet-Boigny's 49 percent.

He called the election a ''masquerade'' of democracy and said his supporters were being rounded up and arrested.

The government denied the charges of fraud.

Abidjan was tense but calm today, with armed soldiers guarding the Interior Ministry. Gbagbo's followers appeared to be awaiting the final results.

In the working-class suburb of Yopougon, opposition militants reportedly barricaded a street in front of their party office, and soldiers dismantled the barricade and broke down the office door. But they went away without making any arrests.

Gbagbo produced hundreds of ballots marked for Houphouet-Boigny, which he said had come from one of dozens of boxes stuffed to rig the elections.

His supporters said they had smashed ballot boxes to halt voting Sunday at several polling stations. They claimed boxes were stuffed with votes for Houphouet-Boigny before the polling started.

In London, prices of cocoa dropped as news spread that Houphouet-Boigny would apparently retain power. Ivory Coast is the world's biggest cocoa producer, and experts say its crop helps determine whether the world market remains in surplus.

Gbagbo said the Democratic Party of Houphouet-Boigny, who has ruled since Ivory Coast's independence from France in 1960, also publicized voting stations that did not exist and set up balloting stations that were not advertised.

There were also reports of disorganization at several voting stations, where ballot papers and electoral lists did not arrive or were delivered hours late.

Interior Minister Leon Konan Koffi called the opposition accusations lies. He told reporters Gbagbo's supporters destroyed ballot boxes to stop elections they knew they would lose.

Konan Koffi said the opposition forced nine voting stations in Abidjan to shut. Gbagbo confirmed his ''party militants'' halted elections in 50 stations of one Abidjan suburb alone, and at numerous other places nationwide.

Government officials first said preliminary results were expected Sunday night, but later said they were delayed by difficulties getting results from remote villages.

Gbagbo said he would decide whether to accept the election results after they were announced, but regardless, he said, the election was worth it.

''Nobody ever thought we would get multiparty democracy while Houphouet- Boigny was in Ivory Coast,'' he said. ''It was time to put a stop to the system of there being just a single candidate.''

Legislative elections are scheduled next month. Some 26 political parties have sprung up since Houphouet-Boigny was forced to legalize political opposition after weeks of rioting led by students and joined by the urban poor.