Born in the industrial Basque town of Elgoibar in a Socialist family, Mr. Otegi joined ETA in the 1970s, when Spain was transitioning to democracy after the Franco dictatorship.

After the Spanish police established that he was involved in one of ETA’s commando units, Mr. Otegi escaped to France in 1977. He was eventually arrested and sent back to Spain to stand trial for the kidnapping in 1979 of the director of a Basque factory of Michelin, the tire company.

Upon completing his prison sentence for the kidnapping, Mr. Otegi switched to politics and won a seat in the regional Basque Parliament in 1995. He eventually took over as one of the leaders and the main spokesman of Herri Batasuna, the separatist party most closely linked to ETA.

The party was outlawed in 2003, and Mr. Otegi’s final sentence was for trying to restart it.

Mr. Otegi was one of the main protagonists in an unsuccessful round of peace negotiations between the Spanish government and ETA that ended in 2007, after ETA killed two people with a bomb at the Madrid airport.

ETA declared a unilateral cease-fire, in 2011, but it has yet to unconditionally hand over its weapons, as demanded by the conservative government of Prime Minister Mariano Rajoy in Madrid.

In his written statement from prison, Mr. Otegi suggested that the government had not made a genuine attempt to dismantle ETA, but that it preferred instead to keep ETA alive as a convenient foil to check the political ambitions of Basque separatists.

Still, the region’s current leader, Iñigo Urkullu from the Basque Nationalist Party, has been treading a far more cautious line, monitoring the standoff in Catalonia but opposing the idea of any unilateral declaration of independence, as the Catalans have been threatening to do.