The trial opened Tuesday of three men accused of helping the jihadists behind the 2017 attacks in Barcelona and a nearby town that killed 16 people.
Held under tight security, the trial began shortly after 0900 GMT at a branch of Spain's National Court in San Fernando de Henares near Madrid.
A total of 235 witnesses will be called during the trial which is expected to run until December 16.
The Islamic State group claimed responsibility for the double attack which also wounded 140 people on August 17 and 18 in Barcelona and Cambrils, a resort 100 kilometres (60 miles) to the south.
While none have been charged with direct responsibility for the bloodshed, the trio are in the dock for helping the attackers, one of whom rammed a van into pedestrians on Barcelona's Las Ramblas boulevard, killing 14, among them two children aged three and seven.
The driver then went on the run, killing another person, but was shot dead by police several days later.
Several hours later, five of the driver's accomplices staged a second attack in Cambrils, ramming pedestrians and stabbing a woman who died of her injuries. All five were shot dead by police.
Inside the courtroom, the three defendants could be seen inside a thick glass box, all wearing masks as prosecutors showed video footage taken prior to the attack.
In one clip, van driver Younes Abouyaaqoub, 22, can be seen joking around with another cell member, saying the explosives would do "a lot of damage", while another showed them putting together an explosive belt.
As the footage played, main defendant Mohamed Houli Chemlal, 23, remained standing, looking on with an air of boredom while Driss Oukabir, 31, and Said Ben Iazza, 27, remained seated and appeared to be watching carefully, an AFP correspondent said.
- 'Terrorist murder' -
Chemlal and Oukabir are both on trial for belonging to a jihadist group, the manufacture and possession of explosives and conspiracy to wreak havoc, while Ben Iazza is charged with collaborating with the group and lending them his van and ID.
Prosecutors have called for a 41-year jail sentence for Chemlal and 36 years for Oukabir, whose brother was one of the attackers and who rented the van used in the Barcelona attack.
But Antonio Guerrero, a lawyer for the AVT terror victims' association said all three should be in the dock for "terrorist murder".
"In our opinion, the fact that they didn't actually take part in the criminal activity of murder does not mean we can't prosecute and convict them," he said as he went into the courtroom.
"In our opinion, they were part of the cell."
Outside the courthouse, which is located inside an industrial warehouse, more than a dozen TV cameras were gathered behind metal barriers and police tape.
"Long live Spain!" yelled a man who drove past in a white van.