PORT-AU-PRINCE, HAITI, OCT. 21 -- A surprise decision last week by a radical priest to run for the presidency and challenge the growing strength of forces tied to the overthrown Duvalier family dictatorship has heightened fears here violence again may stop a Haitian election.

A critic of the 29-year Duvalier regime, the Rev. Jean Bertrand Aristide, entered a crowded field of presidential candidates Thursday, a day after Roger Lafontant, an ex-interior minister, said he will also run in the Dec. 16 election.

Political leaders say the candidacies of Aristide and Lafontant create volatile conditions because the two lead conflicting factions.

Aristide, a political novice, is a radical populist and has supported violence to counter Duvalierist intimidation. Lafontant is a staunch defender of the fallen regime of Jean Claude Duvalier, who was pushed from power after revolt in the streets in 1986. Some of Lafontant's supporters, who blame Aristide for reprisal attacks on former Ton-tons Macoutes -- the Duvaliers' feared security force -- said Aristide would be killed if he continued to campaign.

"We are headed to some sort of catastrophe, a physical confrontation," said Socialist politician Jean-Claude Bajeux, recalling that 1987 elections were aborted after 34 people were slain in attacks at voting places. The attacks were blamed on Duvalierists, although no one has been charged.

The Catholic Church removed Aristide, 37, from his parish post in 1988 for bringing his anti-Duvalierist message to the pulpit. Two months before the expulsion, Aristide's church here in the capital was attacked. Thirteen parishioners were killed. Despite other attempts on his life, Aristide has continued verbal assaults on Duvalierists and foreign governments. He accuses Washington of meddling in Haitian politics.

Aristide's critics and supporters say he is a popular figure in the urban slums. He is said to have announced his candidacy because of the growing political strength of Lafontant since his return from exile in the Dominican Republic on July 7.

"I am a Ton-ton Macoute. I will never deny it. Those who deny it are not men," declared Lafontant, 55, who is alleged by human rights groups to have tortured political prisoners during the Duvaliers' regime. Lafontant dismisses the allegations as baseless and says he is a proponent of democracy.

Branded a public enemy by Louis Roy, president of the Council of State, an interim quasi-legislative body, Lafontant has campaigned to unite powerful remnants of the Duvalier dictatorship, frustrating diplomats and politicians who hoped the army would arrest or exile him.

Warrants charging Lafontant with treason have been issued but his lawyers have fended them off.

Leaders here predict Lafontant will be banned from running in the election because of a constitutional amendment forbidding leading Duvalierists from holding public office until 1996. The electoral council is expected to make its decision on Lafontant Nov. 6. Lafontant said he will support the council's ruling but one Duvalierist said recently he would "get out his guns" if Lafontant was denied participation.

Socialist Bajeux said the electoral council's choice is clear: "If they decide to let him run, they lose credibility. If they decide not to let him, they will lose their lives."