Acting more swiftly and on a larger scale than expected, British forces were reported today to have established a strong land base on a lightly defended corner of the Falkland Islands from which they intend, in the words of a senior government official, "to tighten the ring of steel" around Argentine forces as quickly as possible.

Although the expected counterattack by mainland-based Argentine warplanes inflicted heavy losses on British warships, it failed to prevent about 2,500 British troops from digging in and quickly fortifying themselves, complete with light tanks, antiaircraft missiles, artillery, helicopters and full logistical support, according to the Defense Ministry.

"These are anxious days," Prime Minister Margaret Thatcher told reporters outside 10 Downing St. tonight. "But we have the most marvelous fighting forces in the world. Everybody's behind them. And they're fighting for a just cause."

Today's attacks came after weeks of preparations and preliminary softening-up attacks by the British naval task force around the Falklands while a series of diplomatic negotiations dragged on without a peaceful settlement. A Thatcher aide said today that Britain was now determined to push the Argentine occupation forces off the Falklands.

"There is nothing to negotiate until they get off," the aide said. "I can't imagine a cease-fire now unless the Argentines say enough is enough. The decision has been made. It is taking its course."

According to official accounts and reports on today's action by reporters with the British forces, an accompanying series of lightning raids on both East and West Falkland islands diverted the Argentine forces to allow the British Marines and paratroopers to establish the well-defended bridgehead on East Falkland. From there, they can strike across East Falkland to drive a wedge between the main Argentine forces at Stanley, 60 miles to the east, and Darwin, 20 miles south.

British Harrier jump jets also bombed and strafed the only large concentration of Argentine troops on West Falkland around Fox Bay.

On the heaviest day of fighting in the seven-week undeclared war over the Falklands, Thatcher carried on with a routine Friday of visits to her north London constituency. But throughout the day, and in frequent interruptions of a morning meeting of her "war cabinet" of senior ministers, she was kept informed of the latest developments.

Familiarly terse announcements from the Defense Ministry escalated sharply from spokesman Ian McDonald's noontime two-paragraph description of landings by "a number of raiding parties" from the British task force to Defense Secretary John Nott's late evening statement that British forces "are tonight firmly established back on the Falkland Islands" exactly seven weeks after their April 2 seizure by Argentina.

McDonald also told of the deaths of 21 British troops in a helicopter crash a day or two ago during preparations for today's landings. So far, this is Britain's largest single loss of life during the war.

But Nott, providing no details and refusing to answer questions, said there were more casualties in today's fierce fighting and that five British ships had been damaged, two seriously.

Senior British officials said the military strategy was "to recover the islands at minimum cost" within two weeks if possible by isolating and steadily increasing the pressure on the scattered Argentine garrisons. But today's costly battle, and the prospect of facing similar aerial attacks daily during the campaign, presented Britons a stark contrast with the bloodless recapture on April 25 of the lightly defended island of South Georgia, 800 miles east of the Falklands.

British government ministers repeatedly warned the country in recent days, as invasion became inevitable, that it could not be accomplished without serious casualties. But once the steeply rising death toll becomes known here, it could test the still strong public and parliamentary support with which Thatcher's government went into full-scale battle.

British Defense Ministry sources said at least 1,000 combat and support troops would have been used to establish a major bridgehead at Port San Carlos. British correspondents with the landing parties said "thousands" of troops went ashore. An estimated 4,000 to 5,000 commandos, marines and paratroopers were with the task force before the invasion.

Defense Ministry sources said about 600 of 1,000 troops in a beachhead would be combat troops, supported by about 400 gunnery, engineering, signal, communications, logistics and medical personnel. Artillery, Rapier ground-to-air missiles, blowpipe shoulder-held missiles, Scorpion light tanks, mortars and other weapons and equipment were taken ashore with the troops. British correspondents said a helicopter landing pad was set up.

Defense Ministry sources said landing places for the vertical-take-off-and-landing Harriers also would be secured for what now constitutes the principal British land base for eventual advances on the major concentrations of Argentine forces on East Falkland.

In addition to this main landing, the diversionary attacks elsewhere on both East and West Falkland included raids on isolated Argentine outposts and on ammunition, fuel, food and water stores and naval bombardment of Argentine positions around Stanley, according to the Defense Ministry here. The largest concentration of Argentine forces on West Falkland around Fox Bay also was strafed and bombed by Harrier fighters.

Nott said "several landings and raids were made by our forces in different parts of the Falkland Islands. Some of these forces have remained ashore."

Defense Ministry spokesman McDonald confirmed that raids had been made on both islands and other sources said the purpose was to create as much confusion as possible. Since Argentina expected the offensive, the location of the raids was the key element of surprise.

Britain's naval losses and Argentina's aircraft losses are both severe, accounting for a significant part of their military strength in the war.

Although Britain has never provided numbers, it is believed to have fewer than two dozen warships in the task force of more than 70 ships. Disabling five of the ships, most likely to be frigates and destroyers providing protection for the landing against air attacks, means that perhaps a quarter of the war fleet has been affected.

With British troops now established in stationary positions on the islands, the task force ships will have to move in closer to protect them and the Harriers will have to fly frequent support missions to protect the bridgehead from daylight attacks by Argentine aircraft from the mainland.

But the nine Argentine Mirages, five Skyhawks and two Pucara turboprop ground attack aircraft lost by Argentina, according to Britain, could well amount to 20 percent of Argentina's remaining warplanes since Britain had already claimed to have knocked out at least 19 before today's action.

Britain has lost eight helicopters, mostly from mechanical failure, including the Sea King in which 21 were killed this week. The task force is believed to have started out with slightly more than 50. A military source said this loss of 15 percent or more of the helicopters in the task force will not cause tactical problems for deployment of forces, because the task force has a number of amphibious craft to land troops.

Explaining the mechanical failures, he said the Sea Kings are being used "incredibly intensively in very bad weather under difficult circumstances." Considering the conditions and the amount of flying hours required of the aircraft, he said he did not think that the attrition rate was alarming.

It is now apparent that preparations for this massive operation began at least a week ago, while a U.N. mediation effort was reaching its climax. British commandos raided the Pebble Island airstrip off West Falkland last Friday. The commandos, members of the elite Special Air Services, destroyed 11 aircraft on the strip, seriously reducing the Argentine capability to attack the invasion forces from close range.

Early this week as many as 50 ships of the task force gathered off the Falklands coast to reorganize men and equipment. Yesterday, after the final breakdown of the U.N. negotiations, Rear Adm. Sandy Woodward, commander of the task force, was given the go-ahead for an invasion with timing and operations on the islands depending on tactical considerations and the weather.

Last night, as the assault force approached, there were "ideal" weather conditions, according to BBC reporter Robert Fox who went in with a unit of paratroopers. He said it was "thick cloud, driving rain and a gale."

But overnight, the weather cleared before the predawn attack and the moonlight provided more visibility, causing fears of an Argentine air attack before sunrise. Throughout the three weeks of combat around the islands, however, the Argentine Air Force has not operated during the night and it did not attack until 10 a.m., after the troops had established, and were ready to defend, their bridgehead.