The leader of banned separatist party Batasuna called for peaceful dialogue between all sides to end decades of armed conflict in the Spanish region.
Otegi was expected to launch a formal appeal for a ceasefire
But Arnaldo Otegi stopped short of launching an anticipated appeal for militant group Eta to end its terror campaign.
Spain's government has made it clear that Eta must renounce violence before talks can be resumed with Batasuna.
The party is seen as the political wing of Eta by Spanish authorities.
Speaking to a crowd of 15,000 Basque nationalists gathered in the stadium of San Sebastian, Mr Otegi said he was carrying an olive branch in his hand.
But he made no explicit call for a ceasefire by the militant group - a move widely anticipated by the Spanish media.
In his speech, dismissed and condemned as insufficient by Spanish political parties, Mr Otegi said a referendum should be held on any possible agreement on the future of the Basque country.
"Sometimes it is harder to make peace than to make war," he said.
"Making peace means seeking an alliance with our adversaries and even finally going as far as to seek the cooperation of our enemies," he added.
He also said a double agreement was needed: one between Basque parties, and the other between Eta and the Spanish and French governments.
The latter would require negotiations on demilitarising the conflict.
However, an explosion at a military mountain refuge on Saturday blamed on Eta, the armed group regarded as linked to Batasuna, may undermine the message.
Three soldiers were inside the military shelter in Belagua, in the Navarra region of the Spanish Pyrenees, when two bombs went off but no-one was hurt.
The governor of Navarra said on national radio Eta leaflets had been found by the refuge, but no claim of responsibility has been made by the group.
San Sebastian's mayor, Odon Elorza, and other political groups in the Basque region want the government to take the first step and allow Batasuna back into the political process, our correspondent says.
Batasuna was banned in March 2003 by Spain's previous government over its links to Eta.
Our correspondent says many Spaniards are very sceptical about Batasuna's motives.
More than 800 people have been killed during Eta's 38-year campaign of violence to form an independent Basque homeland in the north of Spain.
Eta has in the past announced a dozen ceasefires, one of which held for 15 months between 1998 and 1999.