The beleaguered government of President Jean-Claude Duvalier imposed a morning-long curfew on Haiti's second-largest city today to prevent the rebellious population from congregating at mass.

The curfew, in Cap-Haitien on the island's northern coast, dramatized the Roman Catholic Church's expanded role as a catalyst for antigovernment agitation and a source of leadership for the impoverished and increasingly disgruntled majority of Haiti's 6 million inhabitants, 90 percent of them Catholic.

Bishop Francois Gayot of Cap-Haitien, speaking by telephone from his bishopric, said residents had planned special ceremonies to mark today's 11th anniversary of his ordination as bishop. But they were prevented from leaving their homes by the 7:30 a.m.-to-1 p.m. curfew announced by officials in government cars driving through the streets with loudspeakers, according to reporters in the city.

Bishop Gayot said government officials gave him no reason for the measure. The prelate heads the Haitian Bishops Conference, which has confronted the government on several issues recently. In addition, he has condemned as "blind and brutal repression" government actions that left three demonstrators killed and several more wounded last Sunday and Monday in Cap-Haitien.

Many residents of Port-au-Prince, the capital, attended mass normally, and the city generally appeared calm for the second day. Reporters who saw a body lying in a street near the port, however, were told that the victim had been shot by Volunteers for National Security, popularly known as Ton-Tons Macoutes. Hospital authorities have reported between four and eight persons killed in the capital during the last three days.

Most of a week of antigovernment agitation has taken place in provincial cities such as Cap-Haitien, Gonaives, Les Cayes and Jeremie, or in smaller towns across the countryside where crowds have gathered to block roads with little or no intervention by Army troops.

Information Minister Adrien Raymond told foreign reporters today not to leave the capital without government authorization, citing concern for their safety. A number of American and other correspondents poured in here after President Reagan's spokesman, Larry Speakes, announced erroneously that Duvalier had fled the country early Friday morning.

Duvalier, vowing instead to remain as "president for life," imposed a state of siege on the country, giving his government the right to impose curfews and take other measures to control the protesting crowds. He also closed down the Catholic Church's Radio Soleil, whose Creole-language news broadcasts and antigovernment statements had made it the most listened to station in the country.

Radio Soleil, which has been closed several times before, marks the most visible sign of opposition to the Duvalier dictatorship from the Haitian Catholic Church. But priests and prelates also have taken the lead in denouncing government abuses in sermons from the pulpit and other public statements, enhancing antigovernment protests in the eyes of many Haitians.

The church's activism has in many ways filled a void left by the absence of effective political opposition. Until recently, political organization was forbidden by law. A change announced last summer authorized political parties for the first time, but only on the condition that their founders swear allegiance to Duvalier.

Many of the government's most active clerical opponents have espoused the doctrine of liberation theology and its commitment to activism on behalf of the poor. Government officials have suggested that in some cases this amounts to subversion.

Priests and Haitian church officials respond that they are only looking after their congregations.