Buenos Aires — The scenes of celebration that greeted Argentina's military invasion of the Falkland Islands, known here as Islas Malvinas, have given way to tension as the country prepares for what the military junta sees as a possibility of war with Britain.
Flags and posters proclaiming ''Islas Malvinas - National Sovereignty'' crowd out balconies and shop windows, but the Argentines, who a week ago thronged outside the presidential palace cheering and laughing, have turned in on themselves. Huddled in cafes, clutching the afternoon editions, they talk about what might happen if the US mediation failed.
Over the last few days the military buildup here has been unprecedented in a country that has never fought a real war. In the main towns that straddle Argentina's South Atlantic coast, civilian hospitals have had their roofs painted with big red crosses so as to identify them as nonmilitary targets to enemy planes.
Local authorities are drawing up civil defense plans and a number of towns have already held practice air raid alerts. Towns like Comodoro Rivadavia and Rio Gallegos, which have sizable airstrips, are the scene of round the clock activity with Hercules and Fokker transport planes providing a shuttle service to and from the Falklands Islands, carrying supplies and troops.
Despite the ominous buildup, a more optimistic note was sounded by Argentina's foreign minister, Nicanor Costa Mendez. ''The danger of war with Great Britain is fading,'' he said on returning from talks in Washington with US Secretary of State Alexander M. Haig Jr. Later Mr. Haig flew on to London, and today he comes to Buenos Aires. Whether this more positive trend takes hold remains to be seen.
While the mood both on the islands and the mainland is one of grim determination rather than joy, there is no sign yet that popular backing for the junta has in any way diminished. If anything the setting sail of the British task force has motivated a political closing of ranks against the ''common enemy.''
At the ceremony swearing in Gen. Mario Menendez as the islands' new military governor April 7, generals were joined by church bishops, trade union officials, and the leaders of the all the main political parties (still officially banned), including the Peronists and the radicals, the two major opposition groupings. They had all volunteered to go to the islands to show their solidarity with the armed forces at a time of ''national need.''
Before the invasion, Argentine President Leopoldo Galtieri was facing growing unpopularity because of the regime's cautious return to democracy and the country's economic problems. The invasion has converted him into a national figure comparable with the ''caudillos'' that have stirred the imagination of the Argentines in different phases of Argentine history.
The junta has reaped much political capital out of the Falklands invasion and knows that immediate withdrawal of its troops or a renouncing of its sovereignty claims over the islands is unacceptable politically. ''We shall defend the islands at whatever cost,'' said Gen. Alfredo Saint Jean, Argentina's interior minister.
The Argentine military buildup and the bellicose rhetoric respond to that emanating from London at the moment. The clear inference from both sides is that before they allow Mr. Haig to mediate successfully between them they must first approach the negotiations from a position of strength.
Diplomats here recognize that with every hour that passes the public posturing of both sides may well get more extreme. But the British Navy is still several days away from reaching Falkland waters and there is still some space for maneuver, diplomats claim.
Each side now recognizes privately that any confrontation over the Falklands will almost certainly involve serious mutual losses in a battle that neither side is convinced of winning.
One view being aired here is that the main thrust of the mediation will be directed at trying to steer both sides away from the sovereignty issue to points where there is a greater chance of common agreement.
One of these is the possibility of some joint cooperation in the development of the islands' economic potential, particularly its oil and fish reserves. Another is some consensus on the future of the islanders. Argentina has already offered an olive branch by saying that they can remain on the islands as British subjects, and will not be subject to the state of siege that is currently enforced on the mainland.
Indeed Buenos Aires is using the fact that invasion was carried out with the minimum of violence - the four people killed were Argentine troopers - as an important negotiating card. The islanders had always argued that they did not want to be part of Argentina because it was a country famed for its disrespect for human rights.
Most observers here believe that the lives of the Falklanders will be less easy to guarantee if and when British troops attack the islands.
''There will be a blood bath if they ever manage to land. The Argentines are in no mood to give up what they believe is theirs,'' said Jorge Bardot, a Spanish businessman who returned April 7 after being evacuated from the islands by the military. Such opinions are likely to be much in the minds of Mr. Haig and company in the crucial days of negotiation ahead.
Meanwhile on the islands themselves the Argentines have moved quickly to consolidate their gains. The amphibious trucks and Navy personnel that were the first to land on the islands on the morning of the invasion are being replaced by soldiers, anti-aircraft guns, and sophisticated radar equipment. The official swearing-in of the islands' new military governor, Gen. Mario Menendez, April 7 was a symbolic gesture, closing the first stage of an occupation that the military junta wants to present to the world as ''irreversible.''
The military is making a concerted effort to establish a political and economic hold on the local civilian population, which until the invasion had always regarded itself as British. They have set up a local TV network linked directly with the mainland and offered each islander a free set. They have replaced local islanders who ran the radio network with Argentines. Before the invasion there was no TV on the Falklands and the radio operated in cooperation with the BBC.
Argentine teachers are replacing Englishwomen who used to tour the islands giving lessons in English language, literature, and history to the families of the local sheep farmers. The Argentines are also trying to introduce the peso to replace the local pound and Argentines are preparing to set up offices in the main town of Port Stanley.