The Argentine government moved quickly today to demonstrate firm political control over the Falkland Islands as signs emerged here that the military hold might not yet be secure.

As joyous endorsements of the bold invasion mounted in newspapers and among political and labor leaders, the military government of Gen. Leopoldo Galtieri appeared to have defused a growing wave of internal opposition that only four days ago sparked the biggest protest demonstrations in the military's six-year-old rule.

Today, political leaders and radio and television commentators continued to proclaim yesterday's invasion of the island group as "a glorious recuperation of national territory." Government officials said they planned to dispatch a junta Sunday to govern the islands and said the Falklands--known here as the Malvinas--had become Argentina's 23rd official province.

Following Argentina's defeat in the U.N. Security Council, which passed a resolution calling for withdrawal of the force from the Falklands, the Organization of American States in Washington announced that Argentina had called for an OAS meeting Monday to hear its foreign minister.

In moves seemingly designed to demonstrate Argentina's determination to keep the territory, held for the last 149 years by Britain, government officials also said that the name of the capital of the islands had been changed from Port Stanley to Puerto Rivero--after an Argentine who defied the British in the last century--and that plans for an Argentine education system were under way.

Although largely symbolic, the measures found a growing wave of political and apparent popular support for the government's actions inside Argentina, where the 4,618-square-mile region of fjords and grasslands has been a fervent issue of patriotism for generations. More recently, evidence that extensive oil deposits lie between the Falklands and the mainland has intensified the interest here and in Britain.

The armed forces action, which had been rumored here since January, sparked speculation that the government was attempting to unify popular support at a time when opposition to the military has been growing significantly.

"They are doing this to cover up the political and economic problems," said a director of a human rights group. "They want to show the country as united--and they have picked the perfect maneuver because in Argentina no one can possibly oppose retaking the Malvinas."

Official announcements and newspaper reports here indicated that violence during seizure of the islands may have been greater than first reported, and that resistance may have continued today.

The government said yesterday that one officer was killed and two members of the marine invasion force were injured in the takeover by the 4,500 Argentine troops. It was also stated that none of the 80 Royal Marine defenders was hurt as all were captured. But an eyewitness account published in the Buenos Aires newspaper Conviccion today said that several of the British marines posted on the large island of East Falkland had escaped into the mountainous interior.

In an article by Salvador Fernandez, who accompanied the invading Argentine troops, the paper also reported that sniper fire in Port Stanley, where about half of the island's 1,800 residents live, had lasted well into the night.

An Argentine Foreign Ministry spokesman said this afternoon that the situation "continued as normal" and that "there have been no changes in activity on the island." As reports of British naval movements toward the Falklands reached Buenos Aires, however, two hospitals in the provincial capital of Mar del Plata, 250 miles southeast of Buenos Aires, were prepared to receive military casualties, and red crosses were painted on their roofs, according to reports from official sources.

The private Argentine news agency DYN also quoted sources as saying that two Argentine submarines had left their bases at Mar del Plata after detecting a British nuclear-powered submarine in international waters off the Argentine coast yesterday. That report was not officially confirmed.

Government comment on activity in the South Atlantic islands 300 miles northeast of the Strait of Magellan was muted today after a series of communiques yesterday insisting on the Argentine claim to the islands' control and promising the Falklands' residents full rights--including the right to retain British citizenship.

Argentina's government-controlled television tonight broadcast a lengthy film report from the islands that showed Port Stanley "in absolute calm and tranquility," as one commentator put it. Residents were shown passing quietly along streets or walking dogs, and children were playing.

In an interview broadcast from the islands, Gen. Osvaldo Garcia, a commander of the invasion force, said Argentina had begun "a process of normalization" on the islands. The general said that while he expected the territory would gradually be assimilated, he added: "It isn't normalized yet. We cannot change the culture and style of living of the people immediately."

"Garcia maintained that the residents "understand that we came here not to conquer but to integrate this territory into Argentina. We need communication with them in every sense of the word."

An unnamed resident, identified as the Catholic bishop in the islands, expressed "a feeling of sadness. But I thank God that none of our civilian people have been hurt."

In Buenos Aires, where blue-and-white Argentine flags hung from roofs and balconies and radio and television stations continued to broadcast patriotic songs and messages, rain fell through most of the afternoon and the streets were quiet. An impromptu series of demonstrations yesterday included a rally in front of the presidential palace of about 10,000 persons.

The popular demonstrations have been filled with blue-collar workers and political party members who only days ago were sharply attacking the government and calling for elections. Among the union leaders who called their membership out to the streets yesterday were directors of the General Confederation of Labor, many of whose members still had not been released from jail after a massive and violent antigovernment rally in the same presidential plaza on Tuesday.

Yesterday, a jubilant President Galtieri, addressing the crowd from the balcony of the presidential palace in the fashion of Argentina's late populist president, Juan D. Peron, promised no retreat from Argentina's assertion of sovereignty.

Sources here who said the military government was determined not to withdraw from the islands noted it had firmly rejected attempts at mediation by U.S. and other diplomatic officials posted here. An extended phone call from President Reagan also was to no avail.

Newspapers today were filled with dozens of advertisements celebrating Argentina's "day of glory," as one put it, and even the Buenos Aires branch of Harrods, the London department store, took an ad saying, "The great Argentine store supports the great national moment."