Police foiled the alleged plan with a series of raids in January 2008 in Spain's second-largest city, after a member of the cell designated to blow himself up got cold feet and alerted authorities, a Spanish prosecutor said.
The 11 men on trial include nine men of Pakistani nationality or origin and two from India. They have all pleaded innocent.
Malik Qadeer, 33, accused of belonging to a part of the cell tasked with making bombs for the attacks, said he is a butane gas delivery man who knows nothing about electronics or chemicals.
"I don't even know how to connect two cables," he told the National Court. Qadeer said he had never even heard the word Taliban until he moved to Spain in 2001, and said he knew nothing of any plot to bomb the metro in Barcelona. "Not even a beast would do that," he said.
His 28-year-old roommate, Shaib Iqbal, also denied any knowledge of the alleged plot.
One of the plan's alleged ringleaders, 40-year-old imam Maroof Ahmed Mirza, refused to testify. Another accused leader, Mohammad Ayud Elahi Bibi, 65, said he practiced a peaceful version of Islam.
After Thursday's session, the trial was scheduled to resume Monday.
When the plot was made public last year, it gave Spaniards a chilling reminder of the Madrid terror bombings of March 2004: packed commuter trains ripped apart by 10 backpack bombs that killed 191 people and wounded more than 1,800.
At the time, a general election was three days away, and Islamic militants who claimed responsibility for the Madrid massacre said it was revenge for the presence of Spanish peacekeepers in Iraq.
In 2008, public transport was allegedly targeted again, a general election was just two months away and the attack was allegedly planned because Spain has troops in Afghanistan, prosecutor Vicente Gonzalez Mota has said.
But there have been doubts as to how close the Barcelona cell actually was to staging what would have been Spain's first suicide terrorist attack.