A prosecutor told a jury yesterday that Ahmed Khalfan Ghailani played a critical role in the al-Qaida cell that bombed two U.S. embassies in 1998, while a defense lawyer for the Tanzanian national insisted he was nothing more than an innocent dupe.

Launching the long-awaited trial of the first Guantánamo detainee to be moved into the civilian justice system, prosecutor Nicholas Lewin and defense lawyer Steve Zissou in opening statements gave the jury radically different views of Mr. Ghailani’s prominence in the plot that killed 224 people, including 12 Americans, and injured thousands at embassies in Kenya and Tanzania on Aug. 7, 1998.