A crowd of more than 10,000 gathered in the velodrome of the town of San Sebastian to give a tumultuous greeting to Arnaldo Otegi after his release last Tuesday.
"We are fighting for the liberty of this country and we will never stop," said Otegi, who has ambitions to become the next leader of the region.
The meeting was a display of Basque independence and culture, featuring songs, dances and the red, white and green of Basque flags.
Many of his supporters queued in pouring rain for hours. Some carried banners demanding the release of around 400 Eta members in jail in France and Spain.
There was also a scattering of Catalan flags, reflecting the separatist movement in Spain's other restive province, Catalonia.
A key figure in the Basque breakaway movement, Otegi, 57, has spent a total of 15 years in jail for separatist acts. But he is also credited with helping to swing the campaign away from the violence.
Otegi was jailed in 2011 for trying to resurrect the outlawed separatist party Batasuna.
The party had been banned in 2003 for being the "political wing" of Eta, blamed for more than 800 killings in its long campaign to create an independent homeland in the Basque country, which straddles part of northern Spain and southwestern France.
Batasuna re-emerged as a legal pro-independence party, Sortu, which campaigns for independence by democratic and non-peaceful means, and Otegi was elected as its secretary general in 2013 while he was still behind bars.
In the 1990s, he also became one of the first Eta members to call for disarmament, earning him comparisons to Gerry Adams, whose Sinn Fein party was once seen as the political voice of the Irish Republican Army (IRA).
In 2006-2007, Otegi was one of the main architects of peace negotiations between Eta and the Spanish government.
The talks broke down after a bombing at Madrid airport in December 2006 that left two dead, an attack carried out by Eta militants opposed to disarmament.
The government then adopted a hard line against Eta, arresting dozens of people with links to the group, including Otegi.
A month after Otegi was sentenced, a weakened Eta declared a "definitive end to armed activity". The group has yet to formally disband or disarm although its last deadly attack on Spanish territory was in August 2009.
Upon his release in his home town of Elgoibar, Otegi urged his supporters to emulate Catalonia, where a coalition of separatist parties won a regional election in September.
Otegi told The New York Times in a written statement from prison that he aims to become the next leader of Spain's Basque country, which will hold regional elections at the end of the year.
He added he will run in March for internal elections to choose candidates for EH Bildu, a pro-independence coalition party.
By Colette Larraburu and Antton Rouget / AFP