SANTIAGO, CHILE, OCT. 30 -- Chile today began enforcing a tough new law that aims to permanently ban Marxist organizations from politics here.

The law includes penalties for news reporting on the views of Marxist groups.

It comes seven months after a law allowing non-Marxist parties to apply for legalization and is intended to prepare the way for the limited return to civilian rule planned by Gen. Augusto Pinochet's government for the end of the decade.

Opposition and progovernment newspapers openly defied the new law by giving full coverage to a press conference by leaders of the country's largest Marxist group, the Communist Party.

The law was also rejected by all opposition political parties and criticized by organizations representing the media.

"This law affects all who are against the current dictatorship, and is an attack on all democrats," said Gutenberg Martinez, national secretary of the country's largest party, the Christian Democrats.

The law was intended to give practical effect to ideological prohibitions in the constitution drafted and implemented by the Pinochet government. These ban organizations that "propagate doctrines that offend the family, propound violence or are based on a totalitarian conception of the state or on class struggle."

Under the law, organizations found in violation will have their property impounded by the state, and their members will be unable to hold public office, work in education or the media or hold posts in labor, professional or student organizations for 10 years. Offenders will also be banned from "exercising the right of political opinion in the communications media" for the same period of time.

Anyone "apologizing for" such banned organizations in the media will face fines ranging from $3,800 to more than $13,000, at current exchange rates. These amounts are doubled for second offenses.

The media will face similar fines, and suspensions for up to 10 editions.

Media representatives have opposed the law on the grounds that it not only severely limits freedom of expression but also creates considerable problems for editors who would have to decide which ideas are ideologically acceptable.

In a statement, the journalists' professional organization condemned the new legislation as "marking a watershed of repression from the point of view of information and our country's political traditions."

The Association of Radio Broadcasters, an organization of station owners, described the law as "harmful to the full exercise of the right of opinion and freedom of expression, as guaranteed by the constitution itself."

The Army's member of the military junta, Gen. Humberto Gordon, said Wednesday that the law was the "best demonstration of the government's will to maintain democracy and liberty in Chile; he who has done nothing has nothing to fear."

Legal parties will be able to field candidates in elections in early 1990 for a congress with limited powers.

These elections are to be the second stage of a planned return to what the government calls "full democracy." They are to follow a plebiscite sometime next year to ratify the military's choice for president once Pinochet's term ends in March 1989.

Newspaper editors were awaiting official reaction today to their reporting of the Communist press conference, at which the party's leaders announced that they no longer oppose their supporters' signing electoral registers opened by the government in preparation for the presidential plebiscite.