According to Spanish newspaper El Mundo, Mohamed Houli Chemlal revealed the cell was planning to strike some of Barcelona’s best-known monuments – including the Sagrada Familia church.
The 21-year-old was among four men taken to Madrid’s National Court to testify for the first time about the plot.
Two of the suspects have been held without bail, one is going to be detained for another 72 hours, and the fourth has been freed.
Chemlal was arrested after being injured in an explosion at a house in Alcanar, southwest of Barcelona, a day before a van drove into pedestrians on the crowded Las Ramblas boulevard.
Officials believe bombs were being prepared at that property for “one or more attacks in Barcelona” – and the explosion killed an imam who is thought to have radicalised the other suspects.
Two of the suspects in court said the imam, Abdelbaki es Satty, was the instigator of the plot – and it has been claimed he was planning a suicide attack in the city.
Police have said 120 gas canisters and traces of explosive TATP were found at the makeshift bomb factory in Alcanar.
The suspects who appeared in court are believed to be the only surviving members of a 12-man terror cell.
Driss Oukabir was also questioned during the hearing.
Although he told prosecutors he had rented the van used in the Las Ramblas attack, he said he believed it was going to used for a house move.
Chemlal and Oukabir – along with Salah el Karib and Mohammed Aalla – have been charged over the attacks, and a Spanish judge is ordering them to be sent to prison until their trial begins.
Aalla is the alleged owner of the car used to hit civilians and a police officer in the town of Cambrils, south of Barcelona.
Chemlal has confessed to his participation in the Islamist cell behind the attack, but the other three arrested men have said they are innocent.
Fifteen people were killed and 120 others wounded in last week’s attacks.
Younes Abouyaaquob, who is suspected of driving the van through crowds of tourists on Las Ramblas, was shot dead by police on Monday.
Oukabir’s younger brother, Moussa, was among the five suspected terrorists killed in a shootout with police in Cambrils early on Friday morning.