In the worst clash since the Games opened, about 500 radical students attacked police with hundreds of firebombs Thursday, chanting "Yankee go home" and threatening to disrupt the Olympics if their jailed leader is not freed.

Opposition leaders and influential newspapers moved to calm growing anti-U.S. sentiment sparked by a series of embarrassing gaffes by U.S. athletes and charges that NBC-TV was biased in its coverage of South Korea.Early Thursday, the International Olympic Committee stripped a silver medal from a Hungarian weightlifter because traces of an anabolic steroid surfaced in his drug test. Eight athletes, four of them medal winners, have been banned from the Games because of drug use, including Canadian sprinter Ben Johnson.

Undisturbed by the ruffling of U.S.-Korean relations, the isolated student turmoil or the drug scandals that have rocked the Games, U.S. female athletes strutted their stuff to win three gold medals in one afternoon.

Jackie Joyner-Kersee, who set a world record last week to take the heptathlon gold, won the long jump over East German rival Heike Drechsler, and her sister-in-law, Florence Griffith Joyner, set a world record of 21.56 in the 200-meter semifinals. She broke it 90 minutes later with a 21.34 and won the gold.

The American women's basketball team, making up in part for the failure of their male counterparts the day before against the Soviet Union, defeated Yugoslavia 77-70 to bring home the gold.

The National Police said security was being tightened in Seoul to ensure a smooth finish to the Games. Authorities - mindful of the 1972 terrorist attack at the Munich Olympics in which 11 Israelis were killed five days before the Games ended - intensified checking of cars and visitors entering sporting events, a spokesman for the Seoul Olympic Organizing Committee said.

Student radicals demanded police release their leader, Oh Yong-shik. They threatened to attack police substations in Seoul, cultural performances tied to the Games and, in the most serious threat, said they might target the men's Olympic marathon on the final day of the Games Sunday.