The biggest athletic show on earth opened today with a record number of nearly 10,000 Olympic athletes from 160 countries marching under a massive security blanket and the jealous glare of communist North Korea. A colorful, 3{-hour spectacle mixing ancient Korean rituals with space-age technology kicked off 16 days of athletic competition at the Seoul Summer Games. The Games will provide a showcase for South Korea and produce the first all-out East-West gold rush since the 1976 Montreal Games. ``I'm glad the sky is so high and so blue,'' Yuk Hee-Jin, a 17-year-old Korean high school student, said on entering the stadium. ``I believe God is blessing my country for its years of hard work and preparations for the Olympic Games.'' After mutual boycotts of the 1980 Moscow and 1984 Los Angeles Games, Soviet bloc and Western athletes strode into Seoul's sun-washed Olympic Stadium in closed ranks to the tumultuous roar of a sellout crowd of 70,000. On the streets of downtown Seoul meanwhile, police arrested about a dozen radical students today for mounting an anti-Olympic protest, witnesses said. But no tear gas was fired to quell the protest and no injuries were reported. At least a billion others watched the Olympics via a 115-nation, worldwide television hookup. NBC alone paid $300 million for television rights to the Games, which cost the South Korean hosts $3.1 billion for stadiums, arenas and various support facilities, all of which were in place well before the start of the Games. By decree of the International Olympic Committee, North Korean television was blacked out. The communist nation had demanded to co-host the Games, but Olympic officials said only one nation can do so. Superlatives of the Seoul Games included: the most athletes (9,633), the most countries (20 more than at Los Angeles), the most gold medals (237) and the most venues (34 for 23 official sports and three demonstration sports). Communist North Korea sparked a mini-boycott that failed to sway its biggest political allies, including the Soviet Union and China. Of the countries following North Korea's boycott, only Cuba ranked as a sports power, while Ethiopia had hopes in the marathon. Both also boycotted the Los Angeles Games. The other no-shows _ Albania, the Seychelles, Madagascar and Nicaragua _ are not regarded as major sports nations. On the eve of the opening ceremonies, North Korea again displayed its wrath, saying in an official Communist Party statement that South Korean President Roh Tae-woo's call for a ``successful opening of the Olympics ... will help intensify the fascist rule, increase the danger of war and mean tolerating the schemes to perpetuate the division of the nation.'' Similar criticism of the Games has come from student radicals in the south, hundreds of whom staged scattered anti-Olympic, anti-government, and anti-U.S. demonstrations in Seoul as the Olympic torch arrived in the capital Friday. But none of the demonstrations came near the heavily guarded Olympic facilities or the torch procession. They also failed to arouse popular support. In one protest at the Kookmin University, about 300 students burned an effigy of President Reagan as well as a U.S. flag. But the anti-Americanism of the radical students found no echo in the Olympic stadium, where the crowd gave one of its loudest accolades to the U.S. team. For South Korea, the Games offered a worldwide showcase for its emergence from a developing country to an Asian industrial power whose exports include computers, automobiles and super tankers. To guard against internal or external threats to the Games, South Korea has marshalled a 120,000-man security force and placed its 650,000-member armed forces on heightened alert. Although not directly involved in Olympic security, the 40,000-member U.S. ground forces were placed on ``increased alert'' and a U.S. Navy carrier task force patrolled off the coast. South Korea has stressed the Games' theme of reconciliation and of ``Harmony and Progress'' _ a theme symbolized in the omnipresent South Korean flag at the center of which is a sunlike disc combining the opposing forces: the red Yin and the blue Yang. The strains of Beethoven's Choral Symphony, lauding the brotherhood of man, resounded through the stadium as the procession of Olympic athletes got underway. From his box which appeared to be enclosed in bullet-proof glass, Roh rose to declare the Games of the 24th Olympiad officially open. Iran, in line with its fundamental Islamic beliefs, was the only country in the procession to have a Korean man, not a woman, carry the signboard bearing its name. The first competitions scheduled for after the ceremony today were women's platform diving, soccer, men's volleyball, men's basketball, boxing and taekwondo, a demonstration sport at these Games. The first gold medal was to be awarded Sunday morning in the women's air-rifle competition. The United States, Soviet Union and East Germany were expected to garner the most medals. As a prelude to the Stadium ceremony today, a flotilla of nearly 400 boats sailed majestically on the nearby Han River, led by a dragon drum ship. The stadium extravaganza, involving 1,500 performers, blended taekwondo demonstrations, age-old Korean folk dances, electronic synthesizers, rock music and sky divers. It also provided the first suspense of the Games in the form of some white doves, who perched on the unlit Olympic torch basin as a platform carrying flame-bearers rose upwards. When the torch was lit, exploding into an inferno, the doves scattered, and it could not be determined if any were killed.