PANAMA CITY, JULY 10 -- Riot police firing buckshot and tear gas today prevented antigovernment protesters from holding a planned nonviolent demonstration that had been expected to draw tens of thousands.

The action by the Panamanian Defense Forces marked a return to violent confrontation with opposition forces after weeks of peaceful protests and steadily declining numbers of police in the streets. It seemed likely to prolong a month-old crisis by intensifying the anger of groups calling for the removal of the top military commander, Gen. Manuel Antonio Noriega.

White-clad demonstrators, most of them of middle-class background, waved white handkerchiefs as they paraded down streets in the central financial district, but police drove them back as they tried to march toward the rally site. While military helicopters buzzed low overhead, the blasts of shotguns and tear gas grenade launchers resounded continuously for many hours.

At midafternoon, stinging clouds of tear gas hung over much of the district. According to preliminary reports, at least three dozen protesters and one policeman were injured and police arrested 50.

President Eric Arturo Delvalle, warning of possible violence, banned today's demonstration in an order issued Tuesday. He also prohibited a pro-Noriega rally, scheduled for yesterday, that organizers said would bring together more than 100,000 of Noriega's sympathizers. That rally was canceled.

The opposition National Civic Crusade, led by business and professional groups, rejected Delvalle's decree as unconstitutional. The show of police force today was seen as an attempt by the government, in which Noriega holds the real power, to reassert some of the president's badly damaged authority, and as a sign that Noriega intends to battle efforts to oust him.

Major bloodshed was averted because both sides showed more restraint than they did in June, when oppositon mobs hurled rocks, and riot police, known as Dobermans, used clubs and rifles freely. Today the Dobermans, behind their shields and gas masks and carrying rifles and night sticks, did not grab demonstrators to beat them, and most protesters ran away when chased.

Nonetheless, the scene was like a battlefield across much of the capital. The Crusade called for marchers to converge from five outlying assembly points toward the financial district's Del Carmen Roman Catholic church. Fully armed regular combat troops stationed at two of the meeting points, El Dorado and Betania, searched and arrested anyone honking a car horn or waving a white handkerchief, the hallmark Crusade signals.

After police riflemen fired randomly down one street, five onlookers who had been lunching in Jimmy's sidewalk cafe fled across the street streaming blood, their faces and backs pocked with dozens of buckshot wounds. By evening, streets citywide were cluttered with barricades of smoky fires and debris, and irate Panamanians banged pots outside their homes as police convoys cruised by.

U.S. diplomats and military officers had been concerned since yesterday that the demonstration could result in killings.

Two Crusade leaders, Eduardo Vallarino and Carlos Gonzalez de la Larza, met at the U.S. Embassy yesterday afternoon with Ambassador Arthur H. Davis at Davis' request. Davis said that the Defense Forces chief of staff, Col. Marcos Justine, asked the ambassador to tell the Crusade that the Defense Forces could not allow the march and would repress it, de la Larza said.

He said Davis expressed fear of bloody violence but did not try to dissuade the Crusade leaders from holding the march.

In a "spot report" dated yesterday and circulated today in the capital, an unidentified officer of the 470th U.S. Military Intelligence Group here wrote that his Panamanian liaison, a Major Valdonero, warned him "bloodshed and death is expected when pro-government forces clash with the opposition."

The report quotes the Panamanian major as saying, "The Panamanian Defense Forces have decided tomorrow is a no-win situation for them. They have therefore made the decision to let the two groups go at it."

However, an unnamed senior U.S. intelligence officer comments at the end of the report: "We agree the Defense Forces are in a no-win situation. But they may be providing a worst-case scenario to official U.S. liaison hoping for some kind of reaction by the U.S. to reinforce their threats and posturing to the opposition." The U.S. officer added that he saw no threat to U.S. personnel in the disturbances.

Archbishop Marcos Gregorio McGrath, meeting with the press before the march began, said he met for several hours last night with Delvalle, the Cabinet, Ambassador Davis and other Panamanian officials in a tense debate about the march. Government officials objected strongly, the Roman Catholic leader said, because their own rally had been canceled.

McGrath noted that the tenor of today's march was more peaceful than the propaganda leading up to the frustrated pro-government event had been. He exhorted today's marchers to avoid violence and be on the lookout for provocateurs.