WARSAW, DEC. 17 -- Gustav Husak, who imposed a harsh communist orthodoxy on Czechoslovakia after the 1968 Soviet-led invasion and steadfastly resisted reform through 18 years in power, resigned today from his post as Communist Party chief, the state news agency CTK said.

Husak, 74, was replaced as secretary general of the party by Milos Jakes, 65, a Central Committee secretary who in recent years has specialized in economic affairs but who earlier oversaw a vast purge that crushed the party's liberal wing after the 1968 invasion. Husak retained his positions as state president and member of the party Presidium.

The resignation represented the first change in the cast of mostly aged East European party leaders who were in office when Soviet leader Mikhail Gorbachev came to power in 1985 and who have often appeared resistant to both the content and style of his drive to revise socialism.

Husak, who suppressed some economic and political reforms in Czechoslovakia that have since been adopted by Gorbachev, has been seen as a principal obstacle to any move by the Czechoslovak party to reverse course and join Gorbachev's reform drive.

Several analysts said Husak's resignation could allow the Prague leadership more freedom to act on policy and more leeway to reconsider the policies of Alexander Dubcek, who created the program of "socialism with a human face" during the period called the "Prague Spring" before the 1968 invasion.

Nevertheless, western diplomats and Czechoslovak dissidents said the change did not, in itself, signal any shift in the Czechoslovak party, which remains dominated at the top by mostly aging conservatives. Jakes (pronounced YA-kesh), during nearly 20 years in senior party positions, has failed to create a strong political profile.

"Any change is good -- but just because it is a change, not because of the people involved," said Jiri Dienstbier, a leader of the Charter 77 movement, Czechoslovakia's leading dissident human rights group. "Jakes is the grayest, the biggest nonperson of any of the possible candidates. So no one can really say what is to be expected from his leadership."

Several observers contacted in Prague today said that the direction of Jakes' new regime would not be clear until it was known what changes he would make in the party and government.

"Any real change in the leadership would be surprising," said one veteran western diplomat. "The question is whether this represents a real change of leadership."

One of Jakes' first tasks -- and a key test -- will be the direction of a "complex program" of economic reforms that has slowly been taking shape in the final months of Husak's administration.

Nominally inspired by Gorbachev's program of perestroika, or restructuring, the Prague version presently foresees a more modest decentralization of state economic administration than that adopted by Moscow, and postpones any concrete action until the end of the decade.

The same meeting of the Central Committee that accepted Husak's resignation today is expected to debate a document on the reforms and adopt a resolution on it Friday.

In his first speech as party leader today, Jakes linked the new program to Soviet initiatives, declaring that "just as Soviet Communists, we too must observe the principle that more democracy means more socialism."

A hint that Jakes may face Soviet pressure to strengthen and speed up the local reform came in the form of a message from Gorbachev, who said pointedly that "we are confident the Central Committee under your leadership will ensure the fulfillment of widescale tasks facing the party," including "the restructuring of the economic mechanism {and} the democratization of the public and political life of the country."

Gorbachev has long appeared impatient with Czechoslovakia's stagnation and uneasy about the legacy of 1968.

During a visit there earlier this year, he offered no explicit praise for Husak or his policies and instead said his talks had focused on "plans and designs for the future."

During the celebrations of the 70th anniversary of the Bolshevik Revolution last month, Soviet historian Georgi Smirnov suggested at a press conference that Moscow perceived a need to reconsider the events of 1968, a remark interpreted by some western analysts as the beginning of a partial Soviet rehabilitation of the Prague Spring that could undermine Husak's regime.

Husak, who came to power in April 1969, eight months after the invasion, answered with a speech that stressed what he said was Gorbachev's support for his leadership and party policies. At the same time, he forcefully restated the official, hard-line Czechoslovak party judgment on Dubcek, which maintains that while his administration initially offered promise, it was taken over by "rightist and antisocialist forces" that "threatened the foundations of the socialist system."

Apart from those fleeting public remarks in Moscow and Prague, there was no evidence available today of what role Gorbachev may have played in Husak's retirement. Nor was it clear why the white-haired leader, who with his failing eyesight and stooped posture offered a vivid image of the current frailty of East European leadership, chose this moment to step down.

There was some speculation that Husak, who will turn 75 next month, was prompted partly by that milestone and by declining health. The fact that he will remain in the largely ceremonial position of president and retain his seat on the Presidium suggested that the change was neither forced nor taken suddenly, diplomats said.

As a choice for successor, Jakes represents neither of the two political tendencies -- one strongly proreform and one ultraconservative -- that seemed to coexist in the 11-member Presidium under Husak. Prime Minister Lubomir Strougal, a strong supporter of Gorbachev in public speeches and the chief architect of Prague's own restructuring package, was often cited as a candidate to replace Husak in the event of a decisive shift by the party toward reform.

In contrast, several senior leaders, notably Presidium member Vasil Bilak, appeared openly hostile to both reform and Gorbachev in public statements this year before they were silenced by a Husak speech last March nominally committing the party to Moscow's new agenda.

Jakes, a Presidium member since 1981, has been in charge of supervising economic policy and management for the party but has not assumed a strong public role either in pressing for economic reforms or opposing them. His dull image suggested to some western and Czechoslovak observers that he may have been a compromise candidate or even a transitional figure.

"It is a crazy solution," said Dienstbier in a telephone interview. "Average people know what Bilak and Strougal stand for, but Jakes has never attracted any attention. He's a born conservative in the sense that he is incapable of inspiring people's imagination."

Two characteristics of Jakes do stand out. One is his Czech origin, in contrast to the Slovak background of Husak, who consolidated his regime in part on building up the long-inferior position of Slovakia in the bipartite federal state.

The other clear mark on the record is Jakes' performance as head of the internal Party Control and Auditing Commission from 1968 to 1977. Although he had been appointed to the post by Dubcek, who he reportedly knew as a fellow Czechoslovak student in Moscow, Jakes supervised the post-1968 purge of the party's ranks.

Between the August 1968 invasion and the party congress in May 1971, an estimated 450,000 people were purged or suspended from the party. Many of the cultural and technocratic elite purged were also fired from their state jobs and forced to emigrate or take menial jobs.

A biography of Jakes issued by the Soviet news agency Tass today made reference to his role after 1968, saying that "he took an active part in overcoming the crisis in the party and in society and in solving the problems that had built up earlier." Jakes, said Tass, "firmly defended the interests of the party and the people . . . in the period of open attack by antisocialist and counterrevolutionary forces."