British Prime Minister Margaret Thatcher called on Argentina today to withdraw its troops from the Falkland Islands within the next few days to avoid a bloody battle for the capital of Stanley.

Thatcher said she would not delay the British military advance toward the capital. But during the short time it will take British forces to prepare for the battle, she said, she hoped that the Argentine military government might agree to a guaranteed, supervised withdrawal, which would amount to surrender.

"No one would be more pleased than I should be if either President Leopoldo Galtieri or the commander of their local garrison should say, 'This is absurd that we should sacrifice our young people in this way and we will not fight further.' " said the prime minister in an interview with The Washington Post.

British forces continued to pound Stanley Wednesday, according to their Royal Marine commander, and British correspondents on the scene said the Marines had closed to within seven miles of the capital. Details, Page A25.

Granting an interview for the first time since British troops landed on the Falklands 12 days ago and began ground operations, Thatcher said of the Argentines, "They must have a lot of fine young men there. I know we have." She added, "You know what happened at Goose Green and Darwin. There was a battle in the early stages and then they suggested there should be a surrender."

Thatcher indicated her hope that the Argentine troops at Stanley might similarly decide to surrender without authorization from Buenos Aires after the battle had begun. Leaflets were dropped by British planes yesterday urging the Argentines to surrender, a senior British official said tonight.

Thatcher said she continued to doubt that Galtieri's government would agree to withdraw without attaching conditions unacceptable to her. "I've always thought it would be unlikely that a dictator would withdraw, although after both sides have suffered it is just remotely possible," she said. "But so far, I've seen no sign of it."

"So far, when they've talked about withdrawal, they've talked about it only in the sense that they want to keep some of the fruits of their occupation," Thatcher said. "And that, of course, is not acceptable to me.

"It's too easy a ploy for the invader who is in occupation of the greater parts of the Falklands to say, 'All right, a cease-fire,' when that still leaves them in occupation of our people."

Asked if she thought Britons would accept a considerable increase in casualties in the battle for Stanley, Thatcher said they "know that to defend liberty and justice, previous generations have lost their lives. They are prepared to see that liberty and justice are defended now, and know that it may mean more loss of life. We hope to minimize that loss of life."

Thatcher said she thought President Reagan would contribute to a "multinational force" to protect the Falklands from a new Argentine invasion after British troops left because Britain agreed to participate in the U.S.-led peace-keeping force that supervised the withdrawal of Israeli troops from the Sinai. She said she also hoped that at least one South American country, which she would not name, would participate.

In Washington, informed sources said the president discussed the Falklands with Thatcher by telephone last weekend and tentatively hopes to meet with her in Paris Thursday, in addition to an already scheduled session in London next week.

The sources said that while Thatcher reiterated her uncompromising stand on Argentine withdrawal, she expressed understanding for the problems U.S. support of Britain is causing U.S.-Latin American relations. The prime minister promised that once Britain regained control of the Falklands, she would consider negotiations with Argentina on the long-range future of the islands and the role that Argentina might play there.

Thatcher, interviewed in her study at 10 Downing Street, responded heatedly when asked about the belief of most Argentines that they have a legitimate claim to what they call the Malvinas.

"That was because they had been brainwashed for 40 years," she said, her voice rising. "It started under Peron.

"What are you going to say to me, 'Just start brainwashing your people that they're entitled to the territory next door and then they've got a claim on them' ? "

Asked about the practical problem of reestablishing relations with Argentina if it were humiliated militarily in the Falklands, Thatcher responded instantly, "Do you want Britain humiliated?"

But later today, in interviews on British television, Thatcher said, "I am not seeking to humiliate anyone at all. I am just asking that the invader return his troops to the mainland. That is not humiliation. It is a restoration of the rule of law."

After Britain repossesses the Falklands, Thatcher said, she intends to "rebuild and rehabilitate and develop" the islands and increase their population with new settlers. "I'm not talking about Argentinians," she said, but others who might be attracted by development of the Falklands' offshore oil and fishing resources.

"It is then my earnest desire that the Falkland islanders, who are British, have the right to self-determination just as in the early days of history we helped many South American countries to be liberated and come to self-determination," Thatcher said. She added that this likely would mean eventual independence for a more populous and better developed Falklands, whose low-income, sheep-farming economy is currently dominated by a British colonial company.

Emphasizing that Britain has given "self-determination and independence" to about 40 former colonies--what she called "quite a large slice of the United Nations"--Thatcher said, "I would like to do that for the Falklands."

But she added that "other people would have to respect that independence," which would require that the security of the Falklands "be guaranteed by a number of countries, of whom I hope the United States will be one."

Thatcher appeared to rule out any future deal with Argentina concerning sovereignty over the Falklands now that British blood had been shed to repossess them. She said the Falkland islanders "will naturally be more hostile to Argentina now, very much more hostile. You've heard the reports from Goose Green and Darwin."

Instead, she stressed, for the first time publicly, the option of giving the Falklands independence after a long period of restored British colonial rule. She said she has already asked a British expert on the Falklands, Lord Shackleton, to update his 1976 government plan for improving the island's airport and roads and developing its mineral resources.

"I believe there is quite a potential for development," Thatcher said, although the Shackleton plan had been shelved for years until now. "The thing that has been holding it up, of course," she added, "has been the quarrel with Argentina. One's purpose will now be to get that development going somehow because there are possibilities there.

"I believe we will get more people there with the development. Quite often when people have been to a place and see the potential, some will like to stay. I think there will be a renewed interest in the Falklands."

While the islanders are still likely to want to maintain the Falklands as a British colony, Thatcher said, she was seeking self-government for them "so no one can accuse us of colonialism."

The prime minister said President Reagan, in his recent statements on the Falklands crisis, has been "absolutely marvelous on one of the supreme things, that aggression must not be seen to pay. If it does, there are 50 to 100 other territories that would be in danger, and I think those of us who lived through our generation and his know that."

Asked about Secretary of State Alexander M. Haig Jr.'s suggestion that she show "magnanimity" in victory in the Falklands, Thatcher said, "I'm not talking so much of victory, but of a return of Argentine forces to the mainland."

"After that is achieved," she said, ". . . we shall start to make special efforts to restore the most friendly relations throughout Latin America."