PANAMA CITY, OCT. 22 -- Hundreds of rifle-bearing combat troops patrolled the streets today to enforce a ban against a major antigovernment demonstration.

Protesters from the National Civic Crusade, an alliance of business, labor and student groups, had planned to gather at 11 a.m. in front of the Del Carmen church in the financial district, but stayed away in the face of the massive display of government force. Shield- and truncheon-carrying riot police stationed at the site dispersed any group larger than a few people, at times with tear gas.

In stifling today's protest, the government further reduced already restricted possibilities for public protest by its opponents and demonstrated its increasing ability to intimidate the opposition by force.

The mainly middle-class opposition, in a political crisis that began in June, is calling for military strongman Gen. Manuel Antonio Noriega to step down. The opposition accuses Noriega of assassination and elections tampering.

Noriega blamed the turmoil on a plot by the United States to overthrow his military rule in order to renege on 1977 treaties giving Panama control of the strategic Panama Canal.

"The people are scared. After five months of arrests and killings, that's a fact of life," said Raul Arias De Para, an opposition activist. At least three Panamanians have been slain by unidentified gunmen during the crisis.

Moments after Arias was interviewed today in the lobby of a hotel here, he was detained by intelligence police who questioned him about The Washington Post and the substance of the interview. Arias has immunity as an alternate legislator in the National Assembly, so he was released.

Main avenues in the city were patrolled by fully armed troops of the 2000 Battalion, a combat force specially trained to defend the canal. At least 10 arrests were reported, but no violence. This was to be the Crusade's first major effort since a general strike called for Aug. 17 failed to materialize.

"We are taking preventive action," said police chief Col. Leonidas Macias. "Any demonstration with the purpose of subversion is prohibited."

Meanwhile, members of the opposition expressed shock at the burning of one of the city's major department stores, the second such fire in three months.

In early July, gunmen torched a luxury department store, the Mansion Dante, as police stood by and watched. That store was owned by the family of Roberto Eisenmann, an exiled opposition leader.

Today, firemen continued to fight sporadic flames amid the smoking skeleton of the downtown Machetazo department store, consumed in fire Tuesday night. Losses at the store, with more than 600 employes, were calculated at $20 million. The main building, half a city block in size, and a 10-story tower were destroyed.

A government investigator, Col. Guillermo Leblanc, attributed the fire to "spontaneous combustion."

Several employes who were at the store or in a building across the street Tuesday night said they heard a small explosion and saw flames dance out a third story window. Moments later they heard a second detonation in another third-floor section and saw flames there, followed by a third explosion.

The Machetazo store is owned by Juan Ramon Poll, a Cuban-born Panamanian citizen. Many employes are Crusade followers, often seen in the streets waving white handkerchiefs, the Crusade's symbol. The store management had received several telephone threats warning the employes to avoid politics, a store administrator said.

Crusade leaders were in hiding after progovernment newspapers reported that at least 17 people were arrested earlier this week on suspicion of sedition. The 17 included at least six members of the opposition Popular Action Party, whose headquarters was raided and closed Tuesday.

The government said it uncovered evidence that opposition protesters had conspired to begin a guerrilla war, whose tactics would include shooting American citizens and blaming it on Noriega's Panamanian Defense Forces. Opposition leaders dismissed these allegations as absurd.

In the strife-torn western province of Chiriqui, gunmen shot and maimed three throughbred horses belonging to the head of a local human rights commission, Rodrigo Miranda. Also in Chiriqui, police raided the headquarters of the opposition Christian Democratic Party and the homes of two of its leaders, Arias said.

In a separate incident, a busload of prisoners traveling from one jail to another through the rural city of Santiago rebelled late yesterday. According to an official communique, four prisoners were killed and 14 prisoners and six guards were wounded, while 17 prisoners escaped.