Prosecutor Nicholas Lewin told the jury Ghailani was an al Qaeda operative "committed to al Qaeda's goal - killing Americans."
Defense Attorney Steven Zissou said Ghailani was essentially a "dupe."
Ghailani is on trial for participating in the terror conspiracy behind the August 7, 1998, twin truck bombings of U.S. embassies in Kenya and Tanzania and the murders of 224 people who died. If convicted, he faces a maximum sentence of life in prison without possibility of parole.
"We will prove that both of these massacres were the work of a single al Qaeda cell," Lewin said, and then walked over to the defendant and pointed at him. "This man, Ahmed Ghailani, was a critical member of that cell."
Lewin told the multiracial jury, seated just Tuesday morning, that Ghailani purchased, with another conspirator, the used Nissan Atlas refrigeration truck "that served as the murder weapon" and bought oxygen and flammable acetlyne gas tanks loaded onto the truck "to increase the fireball and create more shrapnel" and "increase the body count."
Ghailani allegedly fled East Africa the day before a suicide bomber drove the Tanazania truck to the embassy in Dar es Salaam and detonated it at 1039 a.m. on a busy Friday morning.
Earlier that year, al Qaeda leader Osama bin Laden had issued one of his directives to followers to kill Americans, including civilians, anywhere in the world. The terror group's East Africa cell formed in the mid-1990s.
A bin Laden lieutenant and a bomb trainer were two of those cell members on the same flight to Pakistan with Ghailani on Aug. 6, 1998, Lewin said.
Investigators later found an explosive detonator and clothing with TNT residue at Ghailani's residence in Tanzania, Lewin said.
Last week, Judge Lewis Kaplan forbade the trial testimony of a man, Hussein Abebe, who has said he unwittingly sold Ghailani numerous packages of TNT in 1998, because the government learned of Abebe from questioning Ghailani when he was held incommunicado in a secret CIA prison abroad without counsel and was not apprised of his right against self-incrimination.
Defense attorney Zissou told the jury that Ghailani was "someone who hung around, went on errands" for friends who turned out to be in al Qaeda but argued the defendant was not a member of the group. Zissou said Ghailani was "duped innocently to provide assistance for these attacks."
Zissou said Ghailani never went to any terror training camp in Afghanistan before the embassy bombings and there is no evidence that he agreed with bin Laden.
"He did not get indoctrinated," Zissou said. "He is not an al Qaeda member, nor does he share his goals."
He described his client, now 36, but only 24 in 1998, as someone who was immature and was more comfortable playing with children and watching cartoons.
"This case is going to come down to one simple question: did he know? The answer to that question is no," Zissou said. "He was with them but he wasn't one of them."
The government plans to call at least one al Qaeda defector now in the witness protection program, L'Houssaine Kherchtou, to link Ghailani with other conspirators.
Kherchtou previously testified in the 2001 trial that led to the convictions of four men now serving life sentences for the embassy bombings.
The government's first witness, John Lange, who was acting ambassador in Tanzania, reprised his 2001 testimony to describe for jurors what it was like inside the embassy when the attack occurred.