Nearly two weeks after Gen. Augusto Pinochet ordered a state of siege, Chile's opposition movements have been forced to adopt the tactics of the underground even as they seek to organize a decisive public confrontation with the government.

Now accustomed to the perquisites of press conferences, party offices and public gatherings, political organizers of the center and left reluctantly have returned to furtive movements abandoned 18 months ago, when mass antigovernment protests began.

Meetings have been shifted from offices to homes. Messages are delivered by hand rather than telephone. Clandestine leaflets have replaced glossy opposition magazines, and columnists in the daily press have revived the art of elaborate circumlocution.

Some top opposition leaders say they no longer feel safe sleeping in their homes. "The surveillance and the threats are back," said labor leader Manuel Bustos, pointing out the window of his office. "They follow us everywhere we go. They're outside here all day. The other day I went to a meeting and I had two cars of them behind me."

The fear inspired by Pinochet's latest crackdown quickly has become the opposition's most pressing political problem. Yet party activists say the state of siege has offered them a chance to prove that force cannot suppress the political crisis.

"This is the test of fire," said Bustos, who is president of the dissident Union Coordinate. "If we can organize a protest under a state of siege, we will have defeated Pinochet. The regime will have to abandon its strategy and look for another solution. That could mean even greater military action, or another liberalization."

The first opposition attempt to break through the extraordinary restrictions came yesterday, when a dozen opposition leaders toured the La Victoria slum here two days after it had been raided by thousands of troops and police.

Christian Democratic Party President Gabriel Valdes and other party activists spent 45 minutes listening to residents' accounts of how government forces conducted a house-to-house search and detained hundreds of persons.

"We have to go on protesting," Valdes told one woman who complained of President Pinochet's repressive measures. "He doesn't have much time left now."

In addition to such tours through neighborhoods by opposition leaders, the four principal political and labor fronts are seeking to organize two days of nationwide demonstrations at the end of this month. Their object is to disrupt cities with street protests and neighborhood demonstrations one day, then try to shut them down with an informal strike the next. "It will be something that will force the government to react," said a Christian Democrat.

Although the exceptional powers officially were meant to combat terrorism, the government has used them to close six dissident publications, heavily censor the remaining media, restrict freedom of assembly, and conduct raids on party and union headquarters and poor neighborhoods.

The first reaction of many opposition leaders was to drop out of sight. The first measure adopted by the Christian Democratic Party was the suspension of a long-awaited party congress.

"The first reaction was that Pinochet was going to repeat the coup" of 1973, when the military overturned the Socialist government of president Salvador Allende, said Socialist Manuel Contreras. "That created a lot of fear."

What has changed the opposition position during the past 10 days, several sources said, is a perception that the government crackdown has been carefully limited. Although dozens of middle- and low-level activists have been arrested or sentenced to internal exile, authorities have not acted yet against top party leaders and have shown relative restraint in searches and raids.

The avoidance of more drastic measures has led opposition strategists to conclude that support within the government and armed forces for the state of siege is limited and that Pinochet is risking his own political position by maintaining it. "The general impression that exists is that the regime has recognized the high political costs of the state of siege" among its own supporters, said a leftist organizer.

As a result, political and labor leaders have been encouraged to mobilize in the hope of forcing Pinochet to choose between even tougher measures and the abandonment of his current policy. Journalists working for suspended publications have begun distributing a daily newsletter containing information censored by the government. Opposition sources say organizers plan to print 3 million leaflets calling for the two-day protest.

Opposition leaders worry that the government may react to the new mobilization by taking just the measures it has avoided until now, shattering the impression that the crackdown has political limits and liquidating the political movements.

For now, however, the message to the population is that the repression is destined to fail. "How long can this last?" Valdes asked in La Victoria yesterday. "I think the state of siege was a serious error. It will have to end, and when it does, everything will rise up again."