Argentine President Leopoldo Galtieri, facing an angry internal reaction to the surrender of Argentina's garrison on the Falkland Islands, declared tonight that if Britain reestablished its administration on the territory, "there will not be security or definitive peace, and the responsibility will fall on Britain for deepening the conflict."

In a grim 12-minute address on national television, Galtieri did not concede that Argentine troops had surrendered on the Falklands and indicated that his government will not accept Britain's call for a formal statement declaring an end to hostilities in the South Atlantic. He promised that the Falklands "will always be within our reach and sooner or later, we are going to get them."

It was not clear tonight whether Galtieri intended to initiate immediate new military action by Argentina or whether such a move would be supported within his own armed forces. Military sources said Galtieri's military leadership confronted deep divisions within its own ranks even as an angry mob gathered in front of the presidential palace tonight to demand Galtieri's ouster.

Informed sources said that the armed forces were paralyzed by indecision and internal power struggles and suggested that Galtieri, opposed by powerful military leaders was in danger of downfall.

Earlier today, reacting to the news reaching Buenos Aires of the surrender on the Falklands, a crowd estimated by news agencies at between 7,000 and 10,000 people gathered in the Plaza de Mayo, in front of the presidential palace. The crowd shouted obscenities at Galtieri, who was in the palace preparing for his national television address tonight, and called for his ouster.

Gunfire erupted in the plaza area of the city later tonight as angry civilians continued to move through the streets. According to an Argentine news service report, a bus trapped between groups of demonstrators and police was fired upon by unidentified gunmen riding in a Ford Falcon, the kind of car frequently identified with paramilitary forces in Argentina.

Galtieri, who met for more than three hours last night with the top commanders of the Argentine Army, was seen by Argentine political sources to be launching a nationalistic political offensive for his survival in power. In his speech tonight, the 55-year-old Army commander in chief referred to Argentina's military defeat at Stanley only indirectly and attributed it in part to "the military technology of the United States, surprisingly enemies of Argentina and its people."

Britain, Galtieri said, "has the following possibilities: to accept that the situation in the islands will never return to what it was before April 2," when Argentina invaded the territory. In that case, Galtieri said, "We will maintain our willingness to negotiate a route for the recovery of our sovereignty."

If Britain chooses to "restore the colonial regime," Galtieri indicated, Argentina would continue fighting. "Defeatism will be treason," Galtieri said in a harsh voice.

Despite the president's declarations, there was no indication tonight that the Argentine government had yet formally replied to the British note, sent through the Swiss Embassy here, asking for Argentina's agreement to an end to hostilities. The government here has also yet to publicly announce the terms of its forces surrender on the Falklands or even to concede that a surrender had taken place.

The only mention by a government official of what has formally been described only as a cease-fire on the Falklands came from Foreign Minister Nicanor Costa-Mendez, who told reporters while entering the Foreign Ministry this morning that "the surrender is due . . . to the materiel the United States has given to British forces, the sanctions the United States has applied against us and the United States' failure to recognize" the measures favorable to Argentina adopted by American nations bound by the Inter American Treaty on Reciprocal Defense.

In his meeting with the top ranking generals of the Army last night, Galtieri reportedly refused to take responsibility for the defeat in which 15,000 Argentine soldiers were captured in an offensive that ended yesterday with the surrender of Stanley, the islands' capital and said that he intended to continue struggling against Britain.

Galtieri was reported by political sources today to have proposed that Argentina withdraw its ambassador from Washington or even break relations with the United States. But the sources said this proposal, opposed by Costa Mendez, had been overruled by other high-ranking military leaders.

Reports in Buenos Aires said that the Air Force commander, Gen. Basilio Lami Dozo, was opposed to Galtieri's hard-line nationalistic position, and that the Army generals who met with Galtieri last night emerged furious at the president's presentation.

Political sources said that Galtieri, backed by Interior Minister Alfredo Saint Jean and his top political adviser, Gen. Hector Iglesias, could rally significant support for the nationalistic program and perhaps survive the challenge now expected from the angered generals.

Tonight, sources said that Galtieri had assigned the 10 top generals in the Army to prepare a political plan and that the generals were meeting continuously tonight to complete the project. But it was not known if such a plan would be accepted by a wide range of armed forces leaders.

The position of the Navy commander in chief, Rear Adm. Jorge Anaya, also was said to be precarious today by well-informed sources. Anaya met this afternoon with the Navy's admirals, and sources said he could be replaced as commander. Anaya's service is believed to have been chiefly responsible for the planning and execution of the Falklands invasion last April 2, and Anaya was reported to be the strongest proponent within the junta of the action.

One alternative to a straight power struggle within the military would be an attempt to return to a democratic government after the appointment of an interim president by a new junta to be composed of the chiefs of staff of the three services, sources close to the military command said.

If the six-year-old military government could reorganize and present a united front to the nation in the coming days, with or without Galtieri and the other junta members, the country's political situation may stabilize at least temporarily, political leaders said. Change within the government and the return to power of civilian political forces could then proceed in a controlled manner over a period of months or even years.

Despite the calls for an end to military rule and the violent reaction against the Argentine surrender at Stanley, the country's political movements were described by Argentine analysts to be too fragmented and lacking in strong leadership to force Galtieri or the military out of power immediately. "The parties will have a black ball," predicted one political leader. "They will be able to veto any new government that does not have a program of returning to democracy, but they will not determine what happens to the government by themselves."

"No one really knows what will happen," one party official said today. "But one can imagine almost anything. Argentina is an emotional country, and it is badly hurt. The reaction will have to be strong, and perhaps impulsive."

In the past, the military government has planned to remain in power at least until March 1984 and possibly until 1987. A new law regulating political parties has been promised for this month, however, and even before the South Atlantic crisis there were indications that the military would permit elections on the level below president in 1984.

Sources stressed that any change in this plan, involving an ouster of Galtieri, could take time to develop within the armed forces. "The whole when-and-how remains to be arranged," said one well-informed source. "No one knows who would head up a movement, or how it would be done, or who would be president and what the program would be. A consensus has to be reached among the generals of what should be done."

Although some civilian political leaders called today for an immediate ouster of the military government, the nation's media and its most powerful party leaders issued statements that were moderate in tone and that stressed the need for unity.

Antonio Troccoli, a prominent leader of the middle-class Radical Party, said, "We don't have to shame ourselves, or appear cut down, because our combatants have defended the positions that were given to them with honor."