The assassination of President Park Chung Hee has left a political void that is being filled more completely by the military with each passing day of martial law.

Martial law commander Gen. Chung Sung Wha has assumed a particularly prominent role.

The military presence is visible although not especially obtrusive. Soldiers are stationed with automatic weapons in public buildings, and tanks are scattered on a few side streets. But of greater significance than these public displays that began with the imposition of martial law Saturday are the more recent happenings.

The martial law command has taken custody of the Korean Central Intelligence Agency and its once-powerful political apparatus -- from the top level down to the lowly agents who once spied on dissidents for Park.

Gen. Chung's pictures appear in the best positions in Korean newspapers that are censored by his censors. Officers of his command are in charge of the assassination investigation and control what is publicly known about the slayings Friday night of five of Park's bodyguards and the president.

Today, Chung made his first public appeal for confidence in the work of his command. He asked for deep understanding and wholehearted popular support of the martial law command as it tries to overcome the difficulties, and he urged the people to demonstrate their patriotism, dignity and determination in the crisis.

Earlier appeals had been made by Acting President Choi Kyu Hah, who is widely regarded as a figurehead and whose position seems to become less substantial each day. He presides over a group of military and civilian leaders -- Chung, two other military officials and four of five cabinet ministers.

Choi's government today formally named Lt. Gen. Lee Hui Sung, 54, deputy Army chief of staff, as acting director of the KCIA, replacing Park's accused assassin, Kim Jae Kyu. Lee will remain on active military duty under Gen. Chung's command.

The group investigating Park's murder, meanwhile, announced today that "many" persons have been detained in connection with Friday's slayings.

Among them is Park's top aide Kim Kae Won, who escaped unscathed from the room where the killings occurred. He was the only one unharmed, aside from the accused killer Kim.

Whether Chung, the Army chief of staff, has dreams of personal power is the leading question in Seoul today.But as one well-formed Korean observed the other day, the question is somewhat misplaced. Chung's ascent was inevitable under the circumstances, he said, because no other power center was left when Park was killed by his KCIA Chief.

Under Park, no potential political successor was permitted to build a power base. His two parties in the National Assembly were directed by men who took orders from him. His prime minister, Choi, was a largely ceremonial figure. Of the two main opposition leaders of the 1970s, one was imprisoned and is still under house arrest and the other was expelled from the assembly on Park's orders.

In that vacuum, the Army was bound to flourish. It is the most prestigious institution in South Korea, relatively untainted by divisive personal politics. No one yet knows what its ambitions are for the future but the speculation is fascinating for its ironies.Some regard it as a liberalizing institution that may -- if it decides to surrender power -- pave the way to a more open political system.

Curiously, the greatest nervousness over the military's ascent seems to be found among conservatives in Park's own Democratic Republican Party, who never before worried much about authoritarian rule. It is sending out signals to foreign reporters that it would be unwise for Americans to tolerate a military rule for long.

The working assumption is that civilian elections will be held within 90 days of Park's death, in accordance with the constitution. The big question then becomes: what kind of election will it be?

The present, indirect method was embodied in the 1972 constitution, which permitted Park to seek unlimited terms of office. It provided for a National Council of Unification to be elected by voters every six years for the sole purpose of choosing a president.

Reflecting Park's suspicion of partisan politics, it excluded from membership on the council anyone who belonged to an organized political party. The result was a tame, easily manipulated organization reflecting Park's wishes.

Some want to jettison the old system completely and adapt direct election of the president. Before Park's death, that was the battle cry of opposition leader Kin Young Sam, who is expected to take it up again when the political season resumes after Parks funeral on Saturday.

If the old system is retained and the military keeps hands off, the selection of a new president would become a contest for support within the Unification Council that elected Park last year. In that event it is likely that two long-time associates of Park would be the front-runners -- Kim Chong Pil and Chung II Kwon.

Both would be expected to carry on Park's foreign and domestic economic policies. Whether either eiffered seriously with Park on the issues of human rights and political dissent is not known to outsiders.

Kim, 53, is a former Army general who is married to Park's niece and who masterminded the 1961 military coup that brought Park to power.

He was director of the KCIA for two years in the early 1960s and then resigned his military title to organize the Democratic Republican Party, which has ruled the unicameral legistature ever since against a vocal but largely powerless opposition.

He resigned in 1968 after an argument over presidential politics with Park, and some observers felt he had wanted to succeed the president. After two years of inactivity, he returned as the chief advisor to the ruling party at Park's suggestion. He subsequently served two years as Park's prime minister, a largely ceremonial post.

Chung, 61, is a former military leader with considerably experience abroad. He is a graduate of the U.S. Army Staff College and studied at Harvard under Henry Kissinger. He served as South Korea's ambassador to both the United States and France.

He has been foreign minister, prime minister, and speaker of the national assembly, reflecting Park's wishes.

Chung is said to be highly regarded by the Army but lacks a strong political base within the ruling party, even though Park had him named as its chief adviser.