Under heavy pressure from angry politicians of all parties here, the British government today declared its intention to use military force if necessary to regain possession of the Falkland Islands from Argentina, which seized them in a lightning invasion yesterday.

At an emergency session of Parliament in which the government was condemned by all sides for allowing Britain to be so easily "humiliated," Prime Minister Margaret Thatcher announced that a massive, well-armed Royal Navy task force would sail from England and Gibraltar for the Falklands on Monday.

Thatcher also acted to freeze all Argentine assets in Britain and to halt export credits and military sales to Argentina. Britain broke diplomatic relations yesterday.

Argentina flew the governor of the Falklands, the defense force of about 80 Royal Marines and a few women and children on a military aircraft to Uruguay--a nation just across the Plate River from Buenos Aires. The Uruguayans, seeking to limit their involvement, screened off the deportees from the press and said they would be flown on to London.

"The Falkland Islands remain British territory," Thatcher told Parliament. "No aggression and no invasion can alter that simple fact. It is the government's objective to see the islands are free from occupation."

She and her defense secretary, John Nott, said diplomatic efforts to secure an Argentine withdrawal would continue during the two weeks it will take the British armada to reach the islands off the tip of South America. "But if that fails, and it will probably do so," Nott said "we have no choice but to press forward with our plans."

The Royal Navy task force, which will join a nuclear-powered submarine said to be already near the Falklands, will contain nearly 40 warships. Representing about two-thirds of Britain's total naval strength, they include two aircraft carriers, several submarines, two amphibious warships, and numerous missile-firing destroyers, frigates, landing craft, fuel and supply ships. Queen Elizabeth II's second son, 22-year-old Prince Andrew, will be aboard the aircraft carrier Invincible as pilot of a helicopter.

Even with this vast force, Nott acknowledged, Britain could face a costly battle to recapture the Falklands from the Argentine force of a dozen warships and several thousand troops who seized the islands in a few hours early yesterday after at least one gun battle with the lightly armed Royal Marines. The deported governor of the Falkland, South Georgia and the South Sandwich Islands, Rex Hunt, said on arrival in Montevideo, Uruguay, that the Royal Marines comported themselves admirably before being overwhelmed.

"The resolution of this problem will undoubtedly be all the more difficult since the occupation," Nott told Parliament, struggling to be heard over shouted demands for his resignation. "The military problems are formidable, but they are certainly not insoluble."

Although he and Thatcher refused to discuss strategy if the military force is needed, the parliamentary debate covered options from a blockade to force an Argentine withdrawal to a potentially bloody British invasion of the Falklands, which could lead to a larger conflict with Argentina.

Only two speakers during the three-hour debate cautioned against the danger of casualties to British or Argentine forces, the more than 1,800 Falkland Island residents or about 17,000 British nationals living in Argentina, and they were accused of "defeatism."

From the left of the Labor Party, which advocates unilateral nuclear disarmament by Britain, to Thatcher's power base on the right of the Conservative Party, speakers accused the government of failing to take military precautions to prevent the invasion.

Laborite Lord Wigg complained that "we have spent 111 billion pounds on defense since the end of the last war and we can't knock the skin off a rice pudding."

"We appear to have been so woefully unprepared," said Sir Edward DuCann, a Conservative leader in the House of Commons. "It is extraordinary that conventional forces were not deployed earlier. Let's hear no more talk about logistics, how difficult it is to travel long distances," DuCann added--referring to the 8,000 miles of sea between Britain and the Falklands, while the islands are 300 miles east of the tip of mainland Argentina. "I don't remember the duke of Wellington whining about Torres Vedras," a battle in Portugal during the peninsular wars.

In this first Saturday session of Parliament since the Suez crisis in 1956, there also were several references to that earlier Conservative disgrace following an ill-fated foreign adventure, the invasion with France of Egypt in a dispute over control of the Suez Canal. Thatcher, facing the most serious crisis of her premiership, has been accused of too much cautioun when challenged abroad.

Reminding everyone of the pride Thatcher has taken in the sobriquet "iron lady" for her tough anti-Soviet rhetoric, an old parliamentary adversary of hers, renegade Conservative Enoch Powell, said, "In the next week or two, this house and the nation, and the right honorable lady herself, will learn what metal she is made of."

Explanations from Thatcher and Nott about why the government had only recently taken the Argentine threat seriously--and then relied primarily on diplomatic efforts rather than "saber-rattling" to resolve the crisis--were greeted with laughter and shouts of "resign, resign." Even some Conservatives joined in the clamor for Nott's resignation, while the rest sat silent. No one from his party came to Nott's defense.

In an atmosphere reminiscent of the emotional U.S. reaction to the taking of American hostages in Iran, another senior Conservative, Sir Nigel Fisher, stood just behind Thatcher to say, "This is a deeply depressing and distressing episode.

"We have failed to defend the integrity of one of our last remaining colonies. We have been humiliated and the government has much to answer for to this house and the people of the country."

A number of Conservatives threatened to deny Thatcher's government their support in Parliament, unless, in the words of Sir Bernard Braine, "there is firm and effective action to remove the invader and restore the Falkland Islands to British sovereignty."

"This is one of the most critical moments in the history of this country since the war," said another conservative, Patrick Cormack, recalling World War II. "We must not shirk, however distasteful the use of force might be, to try and retrieve the situation. If the government fails to do so, there are many of us on these benches who will have to consider their position."

Warning the government it may have to overcome international pressure against any further use of force in the Falklands, a veteran right-wing Conservative, Sir John Eden, said "the credibility of the government and the honor of the country demand no less."

Nott and Thatcher were reminded that they have been reducing the Royal Navy's surface fleet to save money but recently approved the purchase of the expensive Trident II submarine-launched long-range nuclear missile system to modernize Britain's independent nuclear deterrent. Nott, who has worked closely with U.S. Defense Secretary Caspar W. Weinberger, announced new naval cuts just 10 days ago and bore the brunt of parliamentary skepticism about the Trident decision in a debate last Monday.

Carrington received a cool but less contentious reception in a simultaneous debate in the House of Lords, but he was singled out for strong criticism in absentia in the House of Commons.

He and the Foreign Office have been accused in the past there of appearing too eager to give up Britain's few remaining colonies around the world. "In order to save the prime minister and the government," another right-wing Conservative member of Parliament, John Stokes, said after the debate, "I am afraid that certain heads must roll--including that of the foreign secretary and, I very much regret to say, that of the defense secretary as well."

Government sources insisted, however, that Thatcher was not asking anyone to resign. She gave up a weekend trip to her official country home, Chequers, to direct the government's response to the crisis from Downing Street.