The march follows an upsurge of violence in the northern Basque region. In the two latest incidents, homemade bombs exploded at the house of a socialist councillor, and a building belonging to the French motor company, Peugeot, was attacked.

The marchers in Bilbao are calling on the government to move ETA prisoners - who are currently scattered in jails throughout the country - closer to home.

The government in Madrid has been calling on ETA's political wing - now working in an informal partnership with the new Basque government - to condemn the violence.

But they have refused to do so, saying the violence is a justified way of putting pressure on the government to move more than 500 ETA prisoners to jails in the Basque region.

In recent days, a bar belonging to a conservative Popular Party councillor has been destroyed and a police station damaged by Molotov cocktails.

There have been no casualties, but the authorities are reported to have discovered documents which indicate that ETA's youth wing, Jarrai, is conducting what it calls a low level campaign of violence to destabilise the Popular Party before municipal elections later this year.

The newspaper El Pais reported that documents had been sent out by the umbrella Basque National Liberation Movement (MLNV) to Jarrai members.

"Basque youth should ... extend the social agitation. We are going to mobilise our various sections in different struggles and activities," the newspaper cited the documents as saying.

The leader of the Popular Party, Carlos Iturgaiz, said his colleagues had suffered 58 threats of acts of violence since ETA declared its ceasefire last September.