A harsh crackdown on internal opposition by Gen. Augusto Pinochet is emerging as an attempt to dismantle existing political movements and forcibly impose military plans for Chile's political future, politicians and sources close to the government said today.

In the third day of a nationwide state of siege, security forces continued to raid homes and offices and to arrest opposition political and labor leaders. Church sources said that at least 127 persons were being held late today in Santiago and more than 30 in other major cities, including 65 students arrested during a demonstration yesterday.

Plainclothed and heavily armed civilians believed to work for the government's secret police raided the offices of two labor organizations in Santiago this morning and arrested at least 21 persons, church human rights officials said. They said other arrests were carried out in working-class neighborhoods by agents who arrived with lists of suspects.

Government spokesmen and Interior Minister Sergio Onofre Jarpa continued to describe the arrests and strict controls on the media and freedom of assembly as necessary measures to combat left-wing terrorism.

Pinochet, however, appears to view his initiatives as the means to far more ambitious political ends. Public statements by the 68-year-old general and the nature of the government's initial repression suggest that Pinochet hopes to reverse the results of recent political liberalization and force acceptance of his mandate to rule until at least 1989 and of the military's 1980 constitution.

Political liberalization, Pinochet noted in a television address Tuesday, has only resulted in the reemergence of traditional Chilean political movements, particularly on the left, that have rejected his plans for a military-guided "protected democracy" and made its implementation impossible. The president thus appears to have concluded that only by repressing the center and left opposition movements that have appeared in the last 18 months can he ensure that his own political project is preserved.

"It is precisely in order to safeguard democracy and liberty that today more than ever it is necessary to be inflexible," he said.

Even Chilean politicians who support the military government believe that the new military strategy cannot possibly succeed in the long term. "He can only postpone the problems, and the consequences, when they come, could be very serious," said one well-placed conservative observer, who asked not to be named. "This kind of policy offers no reasonable way out."

However, many politicians here believe that the government crackdown could be extended indefinitely unless Pinochet encounters serious resistance from the public, the government's right-wing civilian base and moderate sectors of the armed forces.

"If there is no response," said Christian Democratic leader Adolfo Zaldivar, "this will be a different country. The democratic forces will be eliminated, and the only alternatives will be violent."

Some resistance to the government's policy is taking shape. Students demonstrated today at the University of Chile's law school following violent disturbances yesterday at the engineering school, and citywide student demonstrations have been called for next week. Leaders of the centrist Democratic Alliance, the leftist Popular Democratic Movement and the National Command of Workers met today to decide whether to call for a national protest or strike at the end of this month.

In addition, the Movement of National Union, an important right-wing group strongly supportive of the government, issued a statement today saying the state of siege "was not necessary" and called on "the silent majority" to "break its passivity and organize in political parties." However, opposition politicians here say that Pinochet's offensive has struck the parties and unions at a moment of vulnerability and left many of their leaders intimidated and disoriented.

"It has been very difficult to formulate a response," one Democratic Alliance leader said.

The principal centrist party, the Christian Democrats, was absorbed in an internal leadership struggle when the state of siege was declared, and government restrictions on meetings may block its plans for a party congress this month. Other parties are also divided, and the opposition as a whole has been unable to reach agreement on a "constitutional pact" uniting groups of the center, left and right on a common proposal for democracy.

Although both the Christian Democrats and the pro-Moscow Communists have strong grass-roots organizations, many politicians question whether it will be possible to organize antigovernment protests or any open political activity in the coming months.