Defense officials, reconstructing the biggest British invasion since the Suez crisis in 1956, said here today that about 5,000 Royal Marine paratroopers and support troops landed on the Falkland Islands yesterday and survived the military operation's most perilous day with "remarkably low casualties."

British officials said there were no casualties among the invading troops or those aboard the two dozen military and civilian ships supporting them. The troops and their weapons, heavy equipment and supplies, the officials said, were ferried by helicopter and landing craft with clockwork precision from before dawn to after nightfall under attack by Argentine warplanes.

Even the requisitioned cruise liner Canberra escaped unscathed from bombs British officials said were dumped randomly in the fjord-like bay called San Carlos where the invasion force landed.

Twenty men have been reported dead or missing from the five warships hit by Argentine bombs and missiles while outside the bay in Falkland Sound dividing the two main islands. One British ship, the frigate Ardent, sank last night after being riddled with missiles, and four others have been damaged but are operable, according to British officials. A destroyer with an unexploded bomb in its operations room and at least two more of the ships were reported today to be still operable.

Five other men were reported dead or missing in crashes of two British helicopters and one Harrier jump jet shot down during the invasion.

Britain's entire naval task force around the Falklands of nearly 100 ships and 25,000 personnel was at its "most vulnerable" during the invasion, according to a senior defense official.

"British casualties were much lower than we feared, quite frankly," he said. "It could have been disastrous if ships with a lot of troops on board had been hit and there were a lot of casualties."

"Two or three bad things and your confidence can slip away," the official added. But he was confident that the British invasion force would now begin moving from its 10-square-mile base on the northwestern corner of East Falkland to begin raids and attacks on Argentine occupation forces.

"We're not going to sit around where we are," said Britain's defense staff chief, Adm. Terence Lewin. "We're going to move--and move fast."

"Yesterday was the day when our forces found their greatest risk," Lewin said. "No merchant ships were damaged and have now withdrawn safely from the area. This was achieved at some cost to the warships protecting them."

In the air, he said, carrier-based British Harrier jump jets "won a complete victory," shooting down eight mainland-based Argentine Mirage and A4 Skyhawk fighter-bombers. The British aircraft suffered no losses in the dogfights, he said. One Harrier, however, was shot down by ground fire in a diversionary bombing raid away from the main invasion.

Altogether, according to the British, Argentina yesterday lost nine Mirages, five Skyhawks, two Pucara ground attack planes and four combat helicopters. Some were hit by missiles from British ships, others by shoulder-held blow-pipe missiles fired by the invasion force from ashore. Some were destroyed on the ground by British commandos.

"We are now established on the ground on the Falklands," Lewin said. "Our confidence remains high." He said the British invasion force will continue to be resupplied by air and sea, while the blockaded Argentine troops soon to come under ground attack "know that every bullet they fire could be their last."

But British Broadcasting Corp. correspondent Brian Hanrahan, who watched the invasion from one of the British assault ships, said in a broadcast tonight, "I think it would be wrong to see this as a fabulous victory. It's the first step in a long process."

Because of the heavy Argentine air attack and the British naval losses, he said, men aboard his ship did not see Friday's fighting as a complete success.

"The atmosphere is one of tremendous relief for almost everybody on board, except for one or two people who have seen it before," Hanrahan said. "This was the first time they had been under fire, and particularly under a modern system, where missiles are fired off with great speed, and causing a tremendous commotion.

"The aircraft are hardly seen. They come over the hillside, they lob a bomb, they fire something, and they disappear," he said. "It's a very frightening experience for a lot of people and they were, I think, shaken by it."

British correspondents with the task force said the troops ashore were now consolidating and putting up perimeter defenses at their 10-square-mile base around San Carlos on the northwestern corner of East Falkland island. They said the first priority was to set up Rapier antiaircraft missile systems to take some of the pressure off the remaining warships standing guard in the bay and Falkland Sound.

A senior British defense official said the troops would next begin moving mostly by helicopter to raid some Argentine positions and try to capture others. Sources said reconnaissance by commandos of the Special Boat Squadron and Special Air Services, who landed secretly on the Falklands weeks ago and radioed reports back to the task force, indicated that many groups of Argentine troops are very young and not well-trained.

"My speculation is that a lot of them will give up quite easily," a senior British official said. "We will make as much headway with them as quickly as possible."

Two groups of about 30 Argentine troops encountered by the invasion force around San Carlos quickly fled, according to Marine Col. Tim Donkin, although some of them apparently managed to shoot down two British reconnaissance helicopters with hand-launched missiles. Sources said that nine of the fleeing Argentines taken prisoner yesterday were "young, cold, wet and miserable."

Sources said the first targets would be small pockets of Argentine troops on the northern part of East Falkland and several hundred of them around the Darwin settlement and Goose Green airfield on a narrow neck of land about 20 miles south of the British land base.

British commandos landed by helicopter, attacked Argentine positions around Darwin and blew up a Puma troop-carrying helicopter on the Goose Green field in a hit-and-run raid during yesterday's invasion, British officials said. The commandos now would be able to help other British troops capture these installations.

The largest concentration of Argentine forces--about 6,000 troops--is dug in around the main town of Stanley, about 60 miles east of the British land base. But British officials said they were relatively "spread out" at many locations around Stanley, some of them separated from each other by water along East Falkland's coast, and could possibly be picked off one by one.

"We don't want a classic military movement of everyone marching on Stanley," one official said. "We want to cause as much trouble as we can moving a lot of men around very quickly at night by helicopter."

The Argentine troops, meanwhile, have lost most of their helicopters, small planes and boats with which they had moved around the Falklands, British officials said. Although the Argentines have numerous armored vehicles, one official said, "We don't see much movement on the ground" because of the rough tracks and soft earth outside the small road system around Stanley.

"We are awaiting the response" of the Argentine garrison around Stanley, Donkin said, "but without helicopters or landing craft, it will be very difficult for them to move."

Reacting to Argentine claims of continuing and successful resistance, Donkin said the invasion "has not been repelled. Nor will it be. We are there to stay."

British defense officials said they were not yet worrying much about West Falkland, where there are two concentrations of Argentine troops adding up to 1,500 at most. Sea Harriers bombed and strafed the largest of the two Argentine bases, around Fox Bay, yesterday.

British officials said West Falkland already had been cut off from the main Argentine bases on East Falkland when Harriers and British warships sunk or destroyed most of the boats the Argentine troops used to move between the two islands.

"They're not causing anybody any trouble at the moment so we're pretty relaxed about it," one defense official said. Referring to British commandos who also landed on West Falkland yesterday, he said, "Some people will be harassing them now and then. People will be going in and out."

Defense officials here and British correspondents with the task force said the invasion force and ships guarding it were not attacked by either ground or air today, although they had been braced for another air attack. Defense officials said the reason for this could be worsening weather around the Argentine bases, a need for Argentine commanders to reassess their strategy, or the heavy losses they suffered yesterday.

By British calculations, only about 30 Argentine Mirages and Skyhawks attacked yesterday, many returning to base to refuel and make a second sortie. This would mean that half were shot down by the British.

"They have suffered very considerable losses," a senior British official said. "How many aircraft are they prepared to lose? Are they prepared to use up their entire Air Force?"

British officials also said there was no sign yet of Argentina's remaining warships moving from near the Argentine coast, where they are a day or two sailing time from the Falklands.

While the Argentine Navy may fear British nuclear-powered submarines patrolling the blockade zone around the Falklands, British officials said they were still worried about the three remaining Argentine diesel-powered submarines.

Only one Argentine sub is believed to be out on patrol now, one official said, but another may be joining it soon. Officials said Sea King helicopters are maintaining a round-the-clock, antisubmarine watch around the task force ships still in Falkland sound.

On the Falklands themselves, Col. Donkin said, "The resistance offered has mirrored exactly what we found on South Georgia," when the British recaptured that island 800 miles east of the Falklands on April 25 without casualties and very little resistance.

"We are not overly optimistic," Donkin said, "but if the level of opposition we have met so far is all that has to be faced, we are very encouraged indeed."

Reuter reported the following from London:

Nineteen SAS commandos, Britain's secret trouble-shooting armed forces elite, were killed in a helicopter crash just before the Falklands landings yesterday, military sources said today. It was the worst blow to the SAS in its 40-year history.

The Defense Ministry did not identify them as members of the SAS (Special Air Services) when it published a list of 21 men killed when the helicopter plunged into the sea.

The sources said the men were being transferred from one ship to another to go on clandestine missions behind Argentine lines when the accident happened.

An aura of invincibility has grown up around the SAS in recent years, based on antiterrorist exploits and undercover work against Republican guerrillas in Northern Ireland.

They were seen on television around the world when black-clad SAS men stormed the Iranian Embassy in London in June 1980 to free 19 hostages held by six gunmen.

The SAS are trained to operate with equal facility in the desert, the jungle or the Northern Irish "bandit country" of North Armagh.