The South Korean opposition is proceeding full tilt with a campaign to amend the country's six-year-old constitution, following a surprise decision by President Chun Doo Hwan to relax his previous stance that the drive was subversive and should be suppressed by force.

On fundamentals, the two sides remain far apart. But the current truce is holding to the point that yesterday several thousand opposition party members were able to parade through Seoul, some of them shouting "drive away military dictatorship," without drawing out the riot police with their batons and tear gas.

The party is now publicly laying plans to open offices in provincial cities for its amendment campaign, which seeks to collect 10 million signatures petitioning for direct election of the president.

Last month, Chun's government was threatening the arrest of anyone who signed or circulated the petitions. It now says it is willing to amend the constitution, although not until 1989.

Chun's shift came a day before Philippine president Ferdinand Marcos was driven out, leading to speculation here about links between the two events. Like Chun, Marcos was an authoritarian ruler who had close ties with the United States.

H.C. Hyun, a governing-party member of the National Assembly, said Chun's goal was reconciliation and the timing was coincidental. "We are two different countries," he said by telephone from Seoul.

It also has been suggested that Chun was unnerved by the opposition's unity and by U.S. criticism of his hard-line approach, and concerned about his image. He is to tour Western Europe in April.

Whether a link exists or not, Marcos' fall has elated Chun's opponents. "Our people are very much impressed and encouraged by the Philippines situation," said Kim Dae Jung, a prominent opposition leader, in a telephone interview today.

Ruling party members also claim some inspiration from it. They say it proves their contention that leaders who stay too long are destined for trouble. Chun, who came to power in 1980, has promised to leave office in 1988. Under the current constitution, which was drafted by a committee appointed by Chun, the vote that year is to be conducted through an electoral college. The opposition New Korea Democratic Party maintains that the electoral college system is open to manipulation by Chun.

The amendment campaign has shaped up for both sides as a direct challenge to Chun's authority. He tried to block it with house arrests and phalanxes of police, but he then suddenly shifted gears on Feb. 24.

Chun invited Lee Min Woo, president of the New Korea Democratic Party, and the heads of two minor opposition parties to lunch and suggested he was willing to change the constitution in 1989, after a transfer of power takes place under the existing constitution. The Olympic Games, another of his concerns, are to be held in Seoul in 1988. Chun has cast the successful conclusion of these two events as an overriding national goal.

He also indicated willingness to tolerate the signature campaign, if it was conducted in an orderly manner and did not become a chain of street demonstrations.

Chun's move has taken the edge off a brewing crisis, in which the opposition party was talking of mass civil disobedience. Dissent is back where Chun wanted it, on the floor of the National Assembly. The two sides have agreed to convene a special session next week to discuss, among other things, a special committee on constitutional revision.

But the opposition has rejected Chun's call to wait until 1989 for the amendment. "To delay democratization is to deny it," Kim Young Sam, a veteran opposition figure, said at a meeting formally launching the campaign.

Kim Dae Jung, who was put under house arrest to block his going to the meeting, said today that the government's new stance constitutes only "technical changes" and that it is working behind the scenes to block the signature campaign. Governmental and many private employes have been threatened with dismissal if they sign the petitions, he charged.

The opposition's own timetable for change calls for a constitutional amendment by the end of this year and presidential elections in 1987. It says it would boycott Chun's 1988 elections.

As for Chun's proposal for constitutional revision the following year, Kim Dae Jung said he believes that the president's idea may be to return to office that year after stepping down in 1988.

Debate promises to be lively in the special session, which is to last 20 days. At the same time, student protest is likely to pick up again, following students' return to campuses early this month at the close of lengthy winter vacations.

Yesterday, about 2,000 students clashed with riot police on four campuses, a comparatively large but by no means unprecedented show of campus opposition.

Another significant development came Sunday with a political sermon by Cardinal Stephen Kim, leader of 2 million Catholics and normally quiet on political affairs.

Noting the agreement to change the constitution, Kim said this meant there must be something wrong with the current one. He expressed hope that the next president would be elected under the new constitution and that the government would be more lenient with student protesters.

Kim's words led again to comparisons with the Philippines, where the larger Catholic Church played a pivotal role in Marcos' downfall.