Ruiz must be returned to France in four months
Jose Javier Arizkuren Ruiz, whose trial began amid fresh violence in the Basque country, had been specially extradited from a French prison to answer the charges in Spain.
But Ruiz was not present for most of Tuesday's proceedings in the Madrid courtroom.
He was ordered to leave after refusing to take his place in the dock and shouting in the Basque language that he refused to take part "in this circus".
'Assasin recruitment'
Hours before the trial got under way, a car bomb believed to be the work of ETA exploded in the Basque town of Sestao, injuring two people.
One of them, who had to have a leg amputated, was named as Eduardo Madina Munoz, a youth leader within the Socialist Party.
At the trial of Ruiz - nicknamed "Kantauri" - Spanish prosecutors are arguing that he recruited Juan Jose Rego Vidal in 1993 to assassinate King Juan Carlos.
The trial is the first in Spain for the 43-year-old former head of ETA's military wing.
Prosecutors allege Ruiz wanted Vidal to shoot first the king, then Prince Consort Felipe and thirdly Prime Minister Jose Maria Aznar.
They also allege that another gunman acting under Ruiz's orders, Jorge Garcia Sertusha, had the king in his line of fire three times in the summer of 1995, but did not pull the trigger because a getaway had not been organised.
Vidal, Sertusha, and Vidal's son Ignacio were convicted and sentenced to terms of up to 37 years in prison in 1997.
The assistant state prosecutor for Madrid, Jesus Santos, has asked the court to sentence Ruiz to 37 years in jail.
The French authorities want Ruiz back in France within four months, to serve out an eight-year sentence for terrorism.
ETA has been fighting for an independent Basque homeland for more than 30 years, killing more than 800 people in the process.