Cleanup crews and local officials are sifting through the charred wreckage of this border town's municipal hall and police station after sympathizers of Mexico's main conservative opposition party left the buildings in smoking ruins a week ago.

"People are thinking of taking up arms," a teen-aged boy, who participated in the night of rioting, said matter-of-factly.

A local journalist, speaking of demonstrations that prevented travel between Eagle Pass, Tex., and this community of 120,000, said, "I didn't believe that they could block the bridge across the Rio Grande , but they did. Now they say they'll blow up the bridge. Maybe they will."

Guerrilla warfare or serious economic sabotage still seem unlikely, but civil unrest linked to the conservative National Action Party's political ambitions has escalated in the last month. Party supporters charged that electoral fraud by the ruling Institutional Revolutionary Party had robbed them of victories in races for local offices.

In response to the unrest, Army troops moved into Piedras Negras and three other municipalities for several days last week to maintain order. Police arrested five opposition activists here on charges that have not been made public.

Many political observers fear more violence this year as National Action, strengthened by Mexico's lingering economic crisis and disillusionment with the governing party, mounts what is expected to be its strongest electoral challenge ever in July. The ruling party, known by its Spanish initials PRI, risks losing its decades-old monopoly on state governorships in races for state offices and a new national legislature.

"The government would be making a very grave mistake if it shuts its eyes to the possibility of violence" if electoral fraud mars coming races, National Action Party chief Pablo Emilio Madero said in an interview Friday evening with Washington Post special correspondent William Orme.

National Action portrays itself as representing clean government and private enterprise, as opposed to what it criticizes as the PRI's corruption and excessive faith in public intervention in the economy. It hopes to add to its gains following a string of victories two years ago in mayoral races in state capitals and in the large border city of Ciudad Juarez, across from El Paso, Tex.

Official figures gave Madero 16 percent of the vote in the 1982 presidential election, compared with 74 percent for the PRI's Miguel de la Madrid. Five other opposition parties, on both the left and right of the PRI, shared the remaining 10 percent.

Madero called the recent violence "a human reaction that we understand but don't approve." However, the party includes hard-liners who welcome popular demonstrations even at the risk of rioting. The party "must stop being a brake on political violence and lead the people toward reestablishing legality and, once and for all, having suffrage respected," Alfredo Arenas Rodriguez, a party leader from Baja California State, said, according to yesterday's edition of the newspaper La Jornada.

The recent protests have been centered here in Coahuila State, where National Action charged that fraud deprived it of victories in several municipal elections on Dec. 2. Both National Action and the PRI swore in mayors after a contested election in Monclova, 150 miles south of here, which like Piedras Negras is a steel-making center and was occupied temporarily by the Army last week. In Nadadores, a town near Monclova, National Action supporters threw the newly installed PRI mayor into an open sewer.

Piedras Negras, surrounded by flat scrubland dotted by occasional ranches, offers examples of several factors commonly cited for National Action's advances. Official figures showed the party losing the Dec. 2 mayoralty race here by only 220 votes out of nearly 22,000 cast, its narrowest defeat ever. National Action claimed, and two local journalists agreed, that official poll-watchers' tallies showed that the opposition party had won by at least 200 votes.

The PRI's biggest headaches are economic, and in Piedras Negras that means mainly inflation and the related steep devaluation of the peso. The Bank of Mexico recently accelerated the daily devaluation of the peso against the U.S. dollar, from 13 Mexican cents to 17.

The PRI's Carlos Juaristi, whose installation as mayor here on Dec. 29 prompted the rioting that night, said that the devaluation "is felt more" in the border area because people mentally measure prices in dollars as well as pesos.

Another reason for National Action's gains in Piedras Negras was disillusionment over what was perceived as intervention in the city's affairs by state and national PRI officials, the local journalists said. Even some traditional PRI local leaders disliked the previous mayor, who was said to have been imposed by Coahuila's governor and by union leaders in Mexico City allied with the PRI, the journalists said.

The rioting began here when police fired tear-gas bombs, and members of a crowd of about 4,000 threw rocks and bottles as the state governor and Juaristi left the town hall after the new mayor's swearing-in ceremony. A 56-year-old man died after inhaling tear gas during the six-hour riot, and hospitals reported treating more than 50 persons for injuries, including 18 policemen.