| Some of the hundreds of thousands of demonstrators march through streets of Madrid |
The march, which included more than 800,000 people by some estimates, passed through two squares that were the scenes of deadly ETA attacks.
Demonstrators chanted slogans against negotiations with the militants, and some protesters carried pictures of people killed by the group.
The protest came just weeks after the Spanish parliament approved the government's plan to open talks with ETA if the group agreed to cease its attacks.
ETA has been blamed for hundreds of deaths in Spain during a decades-long campaign to create an independent Basque state in parts of Spain and southern France.
Some information for this report provided by AP and AFP.