SAN JUAN, P. R., Jan. 1--Roberto Clemente, star outfielder for the Pittsburgh Pirates, died late last night in the crash of a cargo plane carrying relief supplies to the victims of the earthquake in Managua. Three days of national mourning for Mr. Clemente were proclaimed in his native Puerto Rico, where he was the most popular sports figure in the island's history. He is a certainty to be enshrined in Baseball's Hall of Fame. He was only the 11th man in baseball history to get 3,000 hits, and his lifetime batting average of .317 was the highest among active players. Mr. Clemente, who was 38 years old, won the National League batting championship four times in his 18-season career, was named to the All-Star team 12 times and in 1966 was named the league's Most Valuable Player. He was also one of the finest defensive outfielders with a very strong throwing arm. He led the Pittsburgh Pirates to two world championships, in 1960 and 1971, the latter time being named the Most Valuable Player in the World Series. Mr. Clemente was the leader of Puerto Rican efforts to aid the Nicaraguan victims and was aboard the plane because he suspected that relief supplies were falling into the hands of profiteers. The four-engined DC-7 piston-powered plane crashed moments after takeoff from San Juan International Airport at 9:22 P.M. The plane, carrying a crew of three and one other passenger, came down in heavy seas a mile and a half from shore. Coast Guard planes circled the area trying to locate the plane by the light of flares. The wreckage was not found until 5 P.M. today in about 100 feet of water. There was no sign of survivors. Airport officials said the plane crashed after making a normal left bank while climbing after the takeoff. It could not be learned if the pilot, identified as Jerry Hill, radioed that he was in difficulty. Cristobal Colon, a friend of Mr. Clemente who was working on the committee to raise funds and collect clothing for the earthquake victims, said he had driven Mr. Clemente and his wife, Vera, to the airport. Mrs. Clemente did not board the plane. Mrs. Clemente said she was concerned that the plane seemed old and overloaded, but her husband assured her that everything would be all right. When the pilot did not show up until late, she said he told her, "If there is one more delay, we'll leave this for tomorrow." Mr. Colon said Mr. Clemente had insisted on going with the flight to make certain that the supplies got into the hands of the people who needed them. "He had received reports that some of the food and clothing he had sent earlier had fallen into the hands of profiteers," said Mr. Colon. Mr. Clemente had been asked to take part in the collection of funds by Luis Vigoraux, a television producer. "He did not just lend his name to the fund-raising activities the way some famous personalities do," said Mr. Vigoraux. "He took over the entire thing, arranging for collection points, publicity and the transportation to Nicaragua." Mr. Clemente's relief organization had collected $150,000 in cash and tons of clothing and foodstuffs. More money and clothing are still being donated. "We sent a ship loaded with supplies during the week," said a member of the earthquake relief committee. "One of the reasons Roberto went on the plane was to get there before the ship arrived to see the supplies were distributed properly." The baseball star was supposed to be met at the airport by Anastasio Somoza, the Nicaraguan military leader, a friend said. Mr. Clemente's interest in Nicaragua may have been heightened by his experience in managing the Puerto Rican team that participated in the amateur world series held in Managua in late November and December. Sixteen teams participated. The Puerto Ricans took fifth place. News of Mr. Clemente's death plunged Puerto Rico into mourning. Gov. Louis A. Ferre decreed three days of mourning and Governor-elect Rafael Hernandez Colon, who will be sworn into office tomorrow, ordered the cancellation of an inaugural ball and all other social activities related to the inauguration.