PANAMA CITY, AUG. 5 -- Panamanian authorities banned public demonstrations today on the eve of a major antigovernment rally and ordered the arrest of six opposition leaders on charges of plotting to overthrow the military-backed regime.

Panama City Mayor Jilma Noriega de Jurado prohibited "all types of public demonstrations . . . with the evident purpose of disturbing public order."

She said at a news conference that the ban applies to an opposition rally scheduled for Thursday as well as both pro- and antigovernment automobile caravans staged almost daily. She said she called on the Panamanian Defense Forces, which combines police and military, to stop Thursday's demonstration.

The National Civic Crusade, a coalition of groups seeking the ouster of military strongman Gen. Manuel Antonio Noriega, vowed to hold the rally despite the ban. It was expected to be the largest antigovernment demonstration since July 10, when police fired tear gas and shotguns at hundreds of demonstrators.

Attorney General Carlos Villalaz said he issued the arrest orders for the six opposition leaders, including Chamber of Commerce president Aurelio Barria, early today.

Yesterday, government agents raided the offices of the National Civic Crusade, headquartered at the Panamanian Chamber of Commerce. Files and documents were confiscated. Villalaz said the documents showed that opposition activists were plotting to overthrow the government.

The opposition called the attorney general's accusations a "huge lie" and charged that the orders for the arrests of the six men are part of a campaign aimed at silencing critics and scaring people away from Thursday's rally.

Prosecutors said they had no information on whether the arrests had been carried out. Sources at the Chamber of Commerce said the targeted men were staying out of sight while a lawyer worked on bail. Police agents in two cars were spotted parked outside Barria's house early today.

About 50 to 70 reporters and other employes of the opposition newspaper La Prensa, shut down by the government July 26, held a vigil outside the newspaper building to demand its reopening.

Gen. Noriega, who has been accused of corruption and political murder, is considered the real power behind civilian President Eric Arturo Delvalle. He has denied the charges against him.

The wave of opposition protests, which have included the business sector and upper classes, has plunged Panama into its worst political crisis in 20 years. Relations with Washington, which has suspended economic and military aid to Panama, have deteriorated.

The attorney general's office also said it had confiscated from opposition offices telex messages to leaders of Japan, France and the United States calling for suspension of economic support for Panama and pamphlets with instructions for making Molotov cocktails.

A Chamber of Commerce vice president, Raul Mendez, denied in an interview that the Civic Crusade had printed any pamphlets inciting violence.

"The government all along has been accusing {us} of being seditious, of harming the economy. What is new in that?" Mendez said. "Now they are mounting a big fuss, saying they found 'subversive' documents. It is a huge lie to cause fear, spread doubt and increase chaos."

Villalaz listed five men he said he was ordering detained. But another federal prosecutor, Mario Ballesteros, who carried out the raid on the Chamber of Commerce office, added a sixth name.

Also today, the U.S. Embassy issued a statement denying a report in a progovernment newspaper that Ambassador Arthur Davis had been at the Chamber of Commerce office when it was raided.

The report was politically motivated as part of a "vicious anti-American campaign" by the newspaper, Critica, the statement said.