British forces in the South Atlantic remained on full war alert today as officials watched the political upheavals in Buenos Aires for signs that the Argentine junta will comply with London's demand for a declaration ending the Falklands conflict.

On the main island, British troops loaded more than 6,000 Argentine prisoners aboard two transport ships amid reports from journalists there of small-scale rioting by young Argentine conscripts who feared they would be left behind.

Earlier in the day, British diplomatic officials said they no longer believed that the present junta was prepared to end the fighting formally. But they based that assessment on the political situation in Argentina which appeared to be changing rapidly.

Whatever happens in Buenos Aires, an important political phase of the conflict is coming to an end. British diplomatic sources conceded today that it is unlikely the European Community will maintain sanctions on Argentina when community ministers meet this weekend, even if there is no formal end to the conflict.

The Economic Community is eager to restore normal relations with Latin America as soon as possible, the sources say, and the lifting of sanctions is an essential part of that process. While preferring that the Common Market nations await a complete Argentine cease-fire, British officials said that sanctions would start to erode soon in any case.

Before word reached here of the ouster of President Leopoldo Galtieri, Prime Minister Margaret Thatcher told Parliament that her government is still insisting that the junta issue a written pledge that the fighting is over. Privately, sources said political uncertainty in Buenos Aires probably made it doubtful the government would comply.

So while still reveling in the successes of British fighting units, senior officials expect that the aftermath of the Falklands triumph may well be a drawn-out and ragged business that poses formidable political and military problems.

The immediate difficulty for London today remained the fate of what are now estimated to be 10,600 Argentine prisoners held by the British on the islands. The junta informed Britain this morning through Brazilian intermediaries that it would not permit British ships to transport the POWs to Argentine ports, even though British officers say many soldiers are in need of urgent medical assistance. Later word from Buenos Aires indicated the prisoners could now be taken to Argentina.

Thatcher told Parliament that British ships were preparing to transport thousands of young Argentine conscripts to Montevideo, Uruguay, about a thousand miles from the islands. As of this evening, the Uruguayan government, which previously allowed repatriation of prisoners, had not said it would accept the soldiers. According to the British reporters in the Falklands capital of Stanley, a school and a shop were set on fire by rampaging prisoners who broke free from their supervised groups and descended into town early this morning.

British soldiers quickly restored order and no one was reported injured.

The accounts were regarded here as further indication of the desperate condition of the defeated Argentines.

Michael Nicholson of Independent Television News reported from the Stanley airport today on the condition of more than 5,000 prisoners there.

"The airstrip has become their open prison," he said. "Thousands of them standing, sitting, wandering aimlessly around in the rain and biting winds waiting to be herded together and marched the six miles into Port Stanley for that boat ride to the ship.

"Most of them are living in cardboard boxes, or empty oil drums. Some have made shacks out of bits of damaged aircraft and plastic sheeting. I even saw men asleep in the cockpit of a smashed aircraft, anywhere to get away from the wind."

Other reports reaching London indicated that some Argentine units and officers had better rations and shelter than the younger soldiers. British viewers were shown photographs taken earlier in the week of the Argentines throwing down their arms.

The original British plan for the POWs, according to officials, was to transport them to Argentine ports, specifically Comodoro Rivadavia, the nearest mainland harbor. But rejection of this plan by Argentina prompted Britain's search for other routes.

Removing the POWs from the islands is regarded here as vital to the full reassertion of British authority. But as long as Argentina refuses to end the conflict formally, Britain plans to hold a substantial number of the prisoners, in accordance with the Geneva Convention on prisoners of war.

There are unconfirmed reports tonight that as many as 3,000 prisoners, including officers and specialists, might be brought to Britain.

Meanwhile, Thatcher continued to enjoy the benefits of her military success. A Gallup poll released today showed her popularity at a record high and gave her Conservative Party a substantial lead over opposition parties. Labor Party leader Michael Foot's standing dropped to the lowest point of any party chief since the end of World War II.

In Parliament today, opposition politicians attempted to revive some of the prewar economic criticism of Thatcher. One Labor member of Parliament, Ray Powell, arguing for an increase in nurses' pay, told Thatcher, "surely you have enough blood on your hands without having more blood from the National Health Service." He was roundly hooted by Conservatives and the speaker of the House of Commons, George Thomas, chastised him for "personal abuse."