President Leopoldo Galtieri and the two other ruling junta members assembled in military command headquarters here tonight as Argentina braced for a British invasion of the Falkland Islands.

At a lengthy press conference tonight, Foreign Minister Nicanor Costa Mendez said Argentina was considering a new peace proposal that Peruvian President Fernando Belaunde Terry said he has submitted. But Argentine officials said privately there was almost no hope for a last-minute cease-fire in the South Atlantic.

The military's joint chiefs of staff issued one communique during the day reporting a brief bombardment of Stanley, the Falklands' capital, early this morning by a British warship. The report said there had been no other military action today.

Reports from southern port cities said that the seas around the Falklands had been struck by a storm last night that, with winds of more than 40 knots and waves of 16 feet, was believed to be checking major military action by either side.

High-ranking Argentine military commanders have promised a massive air attack on the British fleet as soon as it comes within range, but analysts here say the military will most likely adopt a defensive strategy. The junta is expected to depend on its entrenched troops and guns on the islands and on an air counterattack to drive off any British assault.

Foreign Ministry offices were filled with rumors and confusion this afternoon, and many here seemed to hope that Argentina somehow would be rescued from the battle it has anticipated ever since invading the islands last April 2. One report was passed that President Reagan might personally fly to London to reason with British Prime Minister Margaret Thatcher.

But government sources said the junta members had few such hopes. At his press conference, Costa Mendez bitterly attacked Thatcher and the British government and said, "A war is not inevitable, but it will be inevitable if Great Britain continues being obstructive."

No details were immediately available concerning the new Peruvian proposal. Costa Mendez said Argentina was still willing to negotiate. He called upon the United States to play a role in bringing the two sides back to the bargaining table. "If the United States does not support Great Britain, the road to peace could be very much easier," he said.

Other Argentine sources said earlier that the junta was not inclined to accept further attempts at mediation. The United States had tried through third parties to pressure Argentina to accept an agreement in recent days, one source said.

Argentine officials said they all but abandoned hope for a diplomatic settlement last night, after U.N. Secretary General Javier Perez de Cuellar failed in an effort to send envoys to Buenos Aires and London to arrange a cease-fire. Galtieri accepted the offer in a telephone call with the secretary general, but insisted that envoys be sent to both countries simultaneously, not just to Argentina. Subsequently, Thatcher turned down the idea, sources said.

Costa Mendez denied Britain's claim that the Argentines had rejected the final British peace proposal. He said that Perez de Cuellar had asked both sides to submit proposals separately and had concluded that the two were incompatible. Other Argentine officials said the two sides remained almost as far apart as at the beginning of the crisis seven weeks ago.

Buenos Aires never modified its position that any agreement had to lead at least implicitly to Argentine sovereignty over the disputed South Atlantic territories. For that reason, it rejected British conditions that talks be conducted "without prejudice to claims by either side," as the British had proposed.

Officials here also confirmed that Argentina had proposed that no provision be made for governing the islands after Dec. 31 of this year, and that Argentina had intended to take over the islands once again if there were no agreement by that date.

This position on an enforced cutoff date was nearly identical to a stand that helped lead to the breakdown of the diplomatic mission led by U.S. Secretary of State Alexander M. Haig, Jr. last month.

"We said that because we had to do something to force the British to negotiate," explained one high official. "Under their proposal they could have kept on negotiating without any conclusion for another 150 years."

Despite losses in recent weeks, Argentine forces, at least on paper, retain formidable advantages in any full-scale battle, analysts here said.

It is estimated that 7,000 to 10,000 Argentine troops are on the islands, more than the total number of Marines in the British task force. Most are believed to be deployed around Stanley on East Falkland and Darwin near the center of the island about 40 miles away.

Stanley is regarded as the most important strategic position because it lies around one of the few protected bays permitting a large-scale amphibious assault unhampered by high winds and waves. Photographs and film broadcast on state-controlled television show this bay heavily fortified with artillery, antiaircraft guns and rockets. Troops are pictured behind sandbags in shoulder-deep trenches.

The waters around Stanley and other bay areas reportedly have been mined. A helicopter assault would likely come under heavy antiaircraft fire, analysts say.

Argentine military officials have conceded that there are dozens of coves and inlets on the islands where troops could land without facing immediate resistance. Argentine reports also have said that British commandos may have landed already and could be operating freely without encountering Argentine forces.

Military officials have indicated that Argentina will attempt to defeat the invasion with a counterattack by troops on the Falklands and by air forces based on the mainland.

The key to the counterattack would be air power, the single greatest advantage Argentina holds. Argentina has about 120 high-performance fighters and fighter bombers, including French-made Mirage 3s, Israeli-made Daggers, French Super Entendard fighter bombers and about 66 American-made A4 Skyhawks. In reserve are older British Canberra bombers.

These planes are quartered at bases that run in a line down the southern Atlantic coast from Bahia Blanca, 240 miles south of Buenos Aires, to Ushuaia, at the tip of Argentine territory on Tierra del Fuego. The most important bases are at Comodoro Rivadavia and at Rio Gallegos, which lies 400 miles by air from the Falklands.

Despite these claims and what is apparently real confidence by the Argentine military command that Britain cannot dislodge its forces from their strongholds, analysts here note several weaknesses in the Argentine military position.

If British troops land on an undefended point of East Falkland Island or on West Falkland Island, which is believed to be relatively lightly defended, an Argentine counterattack could be impeded by the lack of reliable transportation. The military forces on the Falklands are reported to be short on helicopters and there is little possibility of moving forces by boat.

Argentina's Navy is unlikely to be of much help and the large number of planes theoretically available to the junta may be deceptive because supplies of some spare parts and weapons is believed short.

Also, the necessity of basing its best planes on the mainland because of damage to Falklands airstrips makes a concentrated attack difficult. Argentine fighters cannot reach the east end of the Falklands, where Stanley lies, without facing a fuel problem that can become critical in extended dogfighting.