Coast Guard search teams yesterday recovered a large chunk of the fuselage of the space shuttle Challenger and several pieces believed to be from the craft's wing, cargo bay door and cockpit.

The Coast Guard also reported spotting thousands of other pieces of the wreckage at sea.

The segment of the fuselage, the central body section of the shuttle, was the most significant discovery in the intensive search operation that was launched immediately after the Challenger exploded 74 seconds into its flight late Tuesday morning.

The fuselage section was hauled aboard the Coast Guard cutter Dallas and was unloaded near Cape Canaveral early today. One ripped section of the ship clearly showed the word "rescue" and an arrow pointing to a section where rescue workers could cut into the spacecraft in an emergency of far less severity.

The Dallas also reported that sonar sweeps have detected a "large object or objects" lying on the ocean floor beneath where the floating fragment of the fuselage was located, according to Coast Guard Lt. Cmdr. Jim Simpson.

Simpson said the Dallas had requested Navy divers to descend to the site for examination. Meanwhile, other ships that can pull large objects from the sea are being sent to help the Dallas retrieve the floating debris.

The Dallas made the reports by radio from well out in the Atlantic, where the search operation is centered.

National Aeronautics and Space Administration officials also confirmed that a 6-by-4-inch bone and tissue fragment, with a piece of fabric attached to it, had washed ashore about 30 miles south of the Cape yesterday. But they said there was no way to determine immediately whether it had come from one of the seven Challenger crew members killed in the explosion.

In Houston, NASA officials spent yesterday preparing for today's memorial service. President Reagan and his wife, Nancy, will attend the half-hour ceremony, along with a contingent of at least 24 senators and 55 House members.

Reagan, speaking before the Conservative Political Action Conference last night in Washington, D.C., said of the shuttle crew: "We know now that God holds them close; and we pray He will comfort their grieving loved ones. And we are aware too of our own duty -- to them and to their memory. We must continue."

As the president was speaking, about 125 people gathered on the Capitol steps to pay tribute to Challenger's crew.

The National Transportation Safety Board sent two of its top investigators to Cape Canaveral yesterday to aid NASA in its probe of Tuesday's disaster. All pieces of the Challenger that are recovered are being brought to a central location and impounded for use in the investigation that is under way.

Yesterday's sightings were the first reports that anything substantial remained intact after the massive explosion that destroyed Challenger and brought the nation's costly and ambitious manned space program to a temporary halt.

Until yesterday, search crews had reported finding only much smaller fragments from the shuttle.

The recovery of any part of the cockpit intact might aid efforts to unravel the final, fatal seconds of the 74-second Challenger flight. However, NASA officials cautioned that there is no cockpit instrument that would have survived intact.

The precise sizes of the pieces discovered at sea yesterday were not immediately known. Simpson said the debris was well offshore, "in the northern part of the search area."

Earlier in the day, several tons of shredded metal, plastic, and other smaller debris from the Challenger were recovered from the calm ocean off the nation's space port.

Despite the reports and the retrievals, no substantial clues have emerged as to what caused the worst catastrophe in U.S. space exploration history.

The search spread to new areas of the Atlantic Ocean. Coast Guard officers reported finding various small pieces possibly from such major elements of the complex shuttle as the solid rocket boosters, the external fuel tank and the orbiter vehicle.

Among the parts recovered by late yesterday were a cone-shaped object with a parachute attached, apparently the nose cone from one of the two solid rocket boosters that were destroyed by radio command about 30 seconds after the Challenger explosion. The boosters, which had broken free and were flying off crazily after the disaster, were blown up to keep them from hitting an inhabited area.

A second conelike object was spotted by a search aircraft, but was not immediately recovered. NASA press briefers said they had no information as to whether official probers had seen the debris and identified it.

Other parts retrieved yesterday include a 10-by-12-foot piece of metal "with wires and gauges" attached, and an "aluminum-like" panel, also with wires attached.

The wreckage is being collected by NASA here and will be examined and analyzed for what it can tell about the mysterious sequence that led to the explosion. All the debris recovered so far has either been pulled from the ocean's surface or had washed up on the quiet beaches of this resort area.

The search area was expanded to 8,000 square miles, an area extending from south Daytona Beach to the south tip of Cape Canaveral, and 50 miles out from the coast. The ocean floor is between 50 and 1,200 feet deep here, a stretch of ocean known to NASA veterans as "the missile graveyard of the world."

The cape, a sandy expanse of palmetto scrub, citrus groves, and resort communities, has been the nation's principal rocket launch site since the 1950s.

NASA spokesman Jim Mizell said investigators will rely heavily on detailed analysis of the telemetry data to determine what went wrong to cause the calamity. The shuttle was radioing information from about 2,000 sensors to ground control in Houston when it exploded.

"Any subtle movement of the orbiter, a sudden movement, or a correction in attitude that the crew itself did not sense may show up way before the explosion," Mizell said.

As the day progressed, searchers moved their operations to the north, following the strong currents of the Gulf Stream, which sweeps northward just a few miles off Florida's beaches.

At the same time, NASA technicians and analysts were reviewing weather data at the time of the explosion to determine the most promising areas to search for wreckage. The shuttle blew up at an altitude of about 48,000 feet, a region where winds are normally at their peak velocity. Some experts said the winds could have carried fragments from the massive fireball dozens of miles.

The search fleet includes two Air Force D130 Hercules planes and a Navy P3 patrol aircraft. Nine helicopters from the Navy, Air Force and Coast Guard also operated yesterday.

The sea search was coordinated by the Dallas, a 378-foot Coast Guard cutter.

NASA took several reporters into the search area late yesterday, but they saw no debris and only the lights of distant ships.

For the journalists gathered here from around the world, the saga of Challenger's last flight has settled into a frustrating wait for facts that arrive few and far between.

The investigators poring over mountains of electronic and other data are operating out of view. Their search for clues to what happened has even included impounding remote cameras that were operating at the launch site, and handwritten notes made by controllers, engineers and technicians throughout NASA's launch and mission control facilities.

Meanwhile, public tours of the Kennedy Space Center, canceled for a day after the explosion, resumed yesterday. The space agency said that "normal work activities resumed" in many parts of the launch complex.

A NASA statement also noted that the three other shuttles -- Columbia, Discovery and Atlantis -- will remain "powered down" through the weekend. At the same time, it said that "work on orbiter thermal protection system tiles will be resumed."

The tiles, which protect the spacecraft from the heat of takeoff and reentry, continued to wash ashore here, the only easily recognizable remains of Challenger. Until the investigators are satisfied that they know what caused that calamitous flight, no shuttle will fly again.