A bomb damaged a downtown auto agency connected to the ruling family early today, killing the night watchman, and three nights ago the government newspaper Le Nouveau Monde was hit by a strong explosion that shattered the windows of the seaside offices.

No one claimed responsibility in either case, but Haitian officials added the blasts to a growing list of largely ineffectual attacks that nevertheless indicate new determination among those violently opposed to the government of President-for-Life Jean-Claude Duvalier.

A half dozen earlier acts of violence during the past 14 months have been attributed by Haitian and U.S. officials to two Miami-based exile groups with little active following in Haiti. The subversives are given little chance of overthrowing Duvalier's 25-year family rule. But some observers say the attacks could provoke a reversal in Haiti's gradual return from the dark terror of the 1960s that gave this island nation a reputation for political repression.

Since Duvalier took over at age 19 in 1971, Haitian and foreign officials here agree, he has moved a long way from the arbitrary violence that his father, Francois "Papa Doc" Duvalier, used to maintain absolute power. The Tontons Macoutes, or political police, still prowl towns and villages, but they have been reorganized and brought under a more stringent control that limits the thuggery for which they became known.

"Things are a lot better now," said a former opposition activist, now a government official. "If you are arrested, there is a security committee that will know about it and decide your case. If you tell a lie, they will beat you. If you are arrogant, they will beat you. But if you tell the truth and have nothing to hide, you have a good chance of coming out okay."

In any case, the basic dictatorial structure remains, guaranteed by six separate police or military organizations whose total manpower triples that of the 7,000-man Army. Overt political opposition is still akin to a state crime, and Duvalier reels his relative liberalization in and out like a skillful fisherman.

After a period of increasingly frank political discussion, for example, more than 100 political activists and journalists were arrested in late 1980 and interrogated at the Army barracks just behind the presidential palace. Two weeks later, 16 were deported without legal proceedings. Others were released, others tried, and, according to the U.S. Lawyers' Committee for Human Rights, others remain in jail without charges.

One of the most prominent was Sylvio Claude, leader of the small Haitian Christian Democratic Party. Following a trial denounced as a farce by human rights groups, he was sentenced to jail. After 19 months behind bars, he received a presidential pardon and was released. Late last year, he was arrested again only to be released after 48 hours and what he said were beatings by police.

Claude now lives quietly in Port-au-Prince. His daughter, also politically active, sought asylum in the Venezuelan Embassy and, Haitian officials report, recently slipped out of the country.

"You cannot exist as an opposition here," said a former critic of the Duvalier government. Apparently convinced of that, an exiled group calling itself the Hector Riobe Brigade has mounted a series of attacks designed to undermine Duvalier's authority and rally potential cohorts.

On New Year's Day, a powerful bomb exploded three blocks from the presidential palace, along the route Duvalier was to have followed for a parade scheduled several hours later. The president was nowhere near but three persons were killed, including the man identified as the bomber.

Five days later, an anonymous caller tipped police to a bomb planted in a bathroom of the Finance Ministry, across the street from the presidential palace. Six days later, a caller reported a bomb in the Central Post Office. Both devices were defused before they went off.

These incidents occurred less than two weeks after authorities in the Dominican Republic, which occupies the other two-thirds of Hispaniola island, arrested a Haitian identified as Hilertaut Dominique and accused him of trying to smuggle a dozen Israeli-made Uzi submachine guns from Miami into Haiti through Santo Domingo.

The FBI has been involved in efforts to combat the subversion since Haitian exiles attempted last July to land a commando team from a light plane that flew from Miami to a mountain road near Gonaives, north of Port-au-Prince.

The five-man group, armed with U.S.-made submachine guns bought in Miami, intended to assassinate the president at his nearby farm, U.S. and Haitian authorities later said. But the efforts flopped and the group flew back to the United States after wounding one American tourist.

In what U.S. diplomatic sources qualified as a gesture to Haiti, a Haitian exile was arrested in Miami Jan. 12 and charged with violating the Arms Export Control Act and the Neutrality Act in connection with the July raid. He was identified as Joel Deeb, son of a former mayor of Port-au-Prince who was an adviser to the current president's father.

Sixteen persons, mostly businessmen of originally Middle Eastern families, were arrested here shortly after the raid. They were interrogated and released after about two weeks, diplomatic sources reported. Deeb is of Lebanese extraction.

The Hector Riobe leader, identified as Jean-Claude Louis-Jean, is reported by diplomatic sources to have been in contact with the Palestine Liberation Organization in search of aid and training.

Haitian authorities were particularly irritated by the July attack because shortly after it failed, a light plane buzzed Port-au-Prince and rained down political tracts promising "Liberation is near." Similar broadsheets showed up after the bombing last January. Another batch has circulated during the past two weeks, still promising liberation.

The Hector Riobe Brigade, named after a victim of the Haitian police, has a membership of about a dozen in Miami, U.S officials say. Some of its members have been involved in the drug trade there, these sources add. The Haitian and U.S. governments, in a joint communique three days before Deeb's arrest, denounced the group's efforts as "terrorism."

Another longtime Duvalier foe, Bernard Sansaricq of Fort Lauderdale, Fla., was put on three years' probation last September by a federal court in Miami after pleading no contest to charges that he violated U.S. neutrality laws with an invasion 14 months ago. His attack fell apart when eight commandos were killed as they landed from a small boat. Sansaricq, who was following with 20 others in a second boat, was intercepted by the U.S. Coast Guard and hauled back to Florida.

The Sansaricq group, the Parti Populaire National Haitien, is regarded as politically conservative. Sansaricq comes from the wealthy mulatto business elite driven from power by Francois Duvalier. There are no signs of cooperation or political unity between the two exiled groups.

About 80 percent of Haiti's population of more than 5 million is illiterate and few Haitians have heard of either opposition group. But a Haitian official, speaking anonymously, suggested that thousands of unemployed youths crowded into Port-au-Prince's slums could provide support if the exile attacks became spectacular enough to attract wide attention.

"Those young people could become a recruiting pool," he added. "It is a real danger."