EXPLOSIONS ROCK WESTERN ENCLAVES IN SAUDI CAPITAL
Four separate overnight bombing attacks strike Western targets in Riyadh, Saudi Arabia, on eve of Secretary of State Colin L Powell's visit there, causing undetermined number of deaths and dozens of injuries; United States Embassy in Riyadh says 44 Americans are injured, some seriously; local press reports deaths of one Saudi, one Lebanese and one Westerner; casualty figures are expected to rise; Powell says violence is likely work of Al Qaeda, saying it bears its hallmarks; says Al Qaeda has been weakened but not destroyed; live pictures from scene attest to power of blasts at apartment compounds that are home to American, British, Italian and other Westerners, as well as to Saudis and citizens of other Middle Eastern countries; Saudi Interior Min Prince Nayef says first three blasts were suicide bombings set off by cars stuffed with explosives driven into targeted compounds; attackers strike after State Dept's extraordinarily specific warning on May 1 that terrorists would soon attack American targets in Saudi Arabia; there was botched attempt by Saudi security services to seize cell that Interior Ministry accused of being linked to Al Qaeda terrorist network; 19 suspected militants escaped, but police seized huge arms cache; photos; map (L)