Despite student-let riots and some unusual signs of broad public disapproval, the South Korean government of President Park Chung Hee seems to be as firmly in command as at any time during his 18-year rule.

In the view of some independent observers in Seoul, Park is even more solidly entrenched than before the last 10 weeks of political unrest. They offer three explanations for his tighter grip on power:

An opposition leader, Kim Young Sam, has lost the privileged position from which he challenged Park's position from which he challenged Park's "authoritarian" ways and is no longer considered a serious threat by Park.

A spate of martial law has squelched, at least temporarily, small but biolent demonstrations by students, who have traditionally been a worrisome element. The revolt did not even spread to seoul, which many had expected would be quick to erupt.

The United States has reaffirmed its military support of South Korea, signaling unequivocally that it will not use its security role there to pressure Park for a more democratic and less repressive rule.

By having opposition leader Kim expelled from the National Assembly, Park suffered a public black eye and earned scorn in the West. The unprecedented mass resignation of opposition assemblymen also earned him no applause. And the ferocious demonstrations in Pusan and Mason last week showed that underground discontent still exists.

But out of those events has emerged no new national leader who seems capable of putting the opposition together.

In the seven years since Park imposed his authoritarian constitution, there have been two prominent challengers. One of them, former presidential candidate Kim Dae Jung, is under house arrest. The government has shut off the visits of foreigh newsmen and diplomats through whom angry pronouncements against Park once were channeled.

The other, Kim Young Sam, who was expelled from the national legislature, is under surveillance and could be arrested at any moment. He has moderated earlier statements in which he seemed to be proposing a coup of sorts, possibly backed by the United States. The government has made it clear that he cannot return to the assembly unless reelected, and elections are five years in the future.He has lost the legislative imminity that protected some of his outbursts against what he called Park's "minority dictatorial 'rule," and his party newspaper has been silenced.

Reasons for the current government crackdown run deeper than fear of Kim Young Sam's criticism or student revolts in two southern cities. Park's enduring fear has been an alliance among four separate streams of dissent -- students, the opposition party, religious minorities, and a large force of low-paid workers who have not enjoyed the fruits of the South Korean economic miracle.

For most of the past seven years, these four elements have acted separately and at no time seemed capable of joining together in support of a common cause. a

In the past year, however, there have been isolated instances of students joining with workers. Under Kim's leadership, the New Democratic Party reached out to embrace those critics of Park who do not usually take part in organized politics.

The most dramatic example of a joining of forces, one which triggered the whole series of events, occurred on August 11. About 200 women employes of a bankrupt company inexplicably took their demands for back pay into the headquarters of the opposition party and staged an emotional sit-in, which was embraced by Kim's party as a just protest.

Government police invaded the building and injured about 100 persons, including not only women workers but some of Kim Young Sam's colleagues and several reporters who had been covering the sit-in. Government spokesmen claim that police acted under pressure and without official sanction after a long, tense night.

The dissidents believe it was a deliberate reprisal ordered by top officials to break up an incipient alliance between Kim's party and workers.

After that brutal incident, Kim made some of his more extreme allegations. He suggested that the United States intervene to curb Park's excesses and, in a phrase that deeply angered government politicians, expressed hope that the Army would remain "neutral" in a showdown.

Government leaders thought it was a call for rebellion. The chairman of the ruling Democratic Republican Party, Park Joon Kyu, said in a recent interview that he is convinced Kim had envisioned a "party of liberation" modeled on that of Cuban leader Fidel Castro.

Kim's expulsion triggered an unprecedented response -- the resignation of all 69 members of Kim's party plus a small splinter group in the assembly. It was evidence of a broader, silent disapproval on the part of ordinary citizens that rarely surfaces in South Korea. Many in Kim's party are displeased with him and on normal occasions are more comfortable with the government than with an apostle of all-out opposition. They were more or less coerced into supporting Kim by their own constituents who, expressed resentment at his ouster and demanded some show of reprisal.

The student riots at Masan and Pusan were less clearly a response to Kim's expulsion. Reports vary on why the first group of Pusan National University students pushed their way into the streets and began destroying police cars and substations. In at least two instances, students said they were protesting the presence of government agents on campus and lack of student participation in college administration.

The demonstrations, which elicited a declaration of martial law in Pusan, were not large by comparison with previsous protests. At most, about 3,000 students marched during the first night of Pusan demonstrations, but they were joined by some local citizens.

The demonstrations were unusual, however, in their violence. Most student demonstrations in South Korea never get beyond the marching stage, and leaders are customarily rounded up before they step off the campus onto a public street. Significantly, two minor protests at Seoul colleges, where police surveillance is massive, never got off the ground.

Western sources in Seoul last week said they expect the government to avoid a massive, nationwide clampdown, unless the trouble spreads to the capital where a fourth of the population lives. They also anticipate that the government will offer a gesture of peace-making toward the opposition party, including an offer to permit most of those who resigned to return to the National Assembly without of course, their leader.