The British counterinvasion of the Falkland Islands began days before Friday's concentrated "D-Day" landings, which British officials had said would never take place.

Ordered during the dying days of diplomatic efforts to achieve a peaceful settlement between Britain and Argentina, the invasion was launched under cover of both darkness and misleading statements of British intentions.

It was accompanied by smaller landings, raids and attacks throughout the rest of the Falklands that were advertised in advance but turned out in many cases to be diversions.

Although most of the world was awaiting a major military escalation by Britain in some form, few expected large-scale landings, which British officials long had claimed publicly were impractical. A senior British defense planner, Marine Col. Tim Donkin, said the invasion "achieved complete surprise" by striking around a bay called San Carlos Water on the northwest corner of East Falkland Island.

By the end of the day, British forces had established a 10-square-mile land base and had set up Rapier antiaircraft missile batteries, sophisticated early warning radar installations and heavy artillery in hills around the base perimeter.

Senior British defense officials, who provided the following account of the military operation, said it began last Wednesday during the last throes of diplomatic negotiations at the United Nations. Scores of ships in the British task force formed up tightly north and east of the Falklands just out of range of Argentina's land-based warplanes. Helicopters ferried troops and equipment from ship to ship for the invasion.

A Sea King helicopter carrying 27 commandos of the secretive Special Air Services crashed during one of these transfers, killing 19 SAS commandos, a Marine and the helicopter pilot. It was Britain's largest single loss since Argentina invaded the Falklands on April 2.

British commandos, who had already landed on the Falklands weeks before for reconnaissance, sabotage and selection of an invasion site, reported to the task force Wednesday that small groups of Argentine troops had moved to two positions around San Carlos Water.

Diversionary attacks also were begun Wednesday. British commandos on the islands attacked Argentine forces in the southern half of East Falkland and on West Falkland.

British warships bombarded other Argentine positions around the Falklands' capital of Stanley and at Mare Harbor, about 30 miles west of Stanley. The British were hoping this would lead the Argentine forces to believe the invasion would come from the south, rather than the north.

Sea Harrier jump jets from a British aircraft carrier continued these attacks Thursday before task force ships began moving after dark to position themselves for Friday's invasion. The two aircraft carriers and some escort ships moved southeast of East Falkland Island, both to confuse the Argentines and to stay out of the range of Argentine warplanes.

But the amphibious landing force--two military assault ships and nearly 20 military and civilian troop and supply ships with escorting destroyers and frigates--turned west along the northern coast of East Falkland. At midnight, the ships moved into Falkland Sound between the two main islands. The main group was preceded by a British frigate that began shelling an Argentine position on Fanning Head, at the entrances to Falkland Sound and San Carlos Water.

British commandos already on the ground drove out 30 to 40 Argentine troops, taking nine prisoners. Other commandos destroyed a Pucara attack plane and blew up several fuel and ammunition dumps at Goose Green airstrip and around Darwin, 20 miles south of San Carlos Water, to divert and prevent resistance from Argentine forces there.

With clockwork precision for hours before dawn Friday, 5,000 Marine commandos, paratroopers and support troops landed from helicopters and landing craft on three beaches around San Carlos Water. During the rest of the day, weapons, artillery, light tanks, construction equipment, food, water and other supplies were ferried continuously from ships crowded into the narrow, fjordlike bay.

"A very neat packing job went on for days beforehand to get it all ashore so quickly," said a senior British defense official. "And it was a good beach to land on. We did not expect much opposition."

British forces landing near San Carlos settlement on the northern rim of the bay found and overran a small company of Argentine defenders. While fleeing, some of the Argentines shot down two British Gazelle reconnaissance planes with shoulder-held antiaircraft missiles.

At dawn Friday, a few Argentine ground attack planes from landing strips somewhere on the Falklands flew in low over the hills to observe and attack the landing forces. At least one was shot down.

At 10 a.m., with most of the British troops ashore and establishing a large bridgehead encircling an eight-mile span of San Carlos Water, the first of at least three waves of mainland-based Argentine warplanes attacked.

During the day, Harriers shot down eight Argentine planes, British officials said. Others were shot down by missiles from the warships and shoulder-held launchers used by troops ashore. The British later claimed that 14 of about 30 Argentine planes participating in the day-long raids were shot down (six other Argentine aircraft were reported destroyed during the day).

Argentine planes that got through to attack the landing force, Col. Donkin said, had difficulty maneuvering in the narrow inlet and dropped their bombs "indiscriminately" into the water, missing but menacing the British ships, including the converted liner Canberra.

But Argentine planes had more room to maneuver over Falkland Sound, Donkin said, and some were able to hit their targets cleanly. Three warships were damaged but remained operational, British officials said, including a destroyer with an unexploded bomb in its operations room that was defused yesterday.

But a frigate was seriously damaged in Friday's bombing raids and another, the Ardent, was set afire after being hit by about 15 missiles from two or three Argentine planes. Sitting alone further south in Falkland Sound shelling the Goose Green area, the Ardent was attacked by several waves of Argentine planes. All but 22 of the estimated 190 crewmen of the Ardent were rescued before the ship sank late Friday or early Saturday, British officials said.

Scorpion light tanks, which landed with the invasion force, were used to push inland and defend the beachhead Friday, and Harriers raided an airstrip just west of Stanley, destroying two Argentine troop-carrying helicopters. One of the Harriers was lost, apparently shot down.