It is easy to warm to Arnaldo Otegi, which must be one of the secrets of his success. He looks younger than his 40 years, and gets away with slightly spiky hair and a silver ear stud. There is a glint in his eye, a sense of fun that is - or at least was - unusual in the leaders of the deadly serious radical party which reflects the views of ETA.

Otegi lists his leisure activities as hill-walking and mushroom-gathering. He cannot have had much time for hobbies in the last year, which has seen him advance a Basque peace process so fast that his opponents often seem three steps behind him.

Last December he was a little-known public representative for Herri Batasuna, when the entire previous central committee was jailed for collaboration with terrorism. With two other key figures, he has brought the party out of a bitterly entrenched conflict, and opened a dialogue which led to the ETA ceasefire.

Charm and mental agility are not his only attributes. There is a much tougher side to him. By his own admission his personal history of membership of ETA, exile in France and jail in Spain has been invaluable in bringing his supporters with him into uncharted territory.

His irreverent approach to this subject also marks him out from his predecessors. "Yes, having been in ETA and being an ex-prisoner is a plus, which seems seriously stupid to me at times. You get all sorts in both groups, after all. That kind of mystification worries me. But commitment counts, and gets you respect. Another person, not having passed through ETA, and saying what I say now, would be a lot less tolerated. Ten years ago, they would have been crucified."

He is more orthodox about the relationship between the radical nationalist movement and ETA. Herri Batasuna has never been controlled by ETA, he says, but "the moral authority of the example of ETA's struggle is evident".

It is hard for anyone outside the movement to see moral authority in ETA's catalogue of assassinations. Otegi, however, insists that he will never publicly criticise an ETA action. What Arnaldo Otegi is saying in public now is that ETA's objective of Basque independence can be achieved without firing a single shot.

What is more striking is that he says that any attempt to impose such independence, until it is democratically demanded by an ample majority of the Basque people, would be "suicidal" for nationalism. A big factor in Herri Batasuna's new politics was the Irish peace initiative. "Ireland was a mirror for us," he says, "and so was the republican movement. Negotiation was always regarded here in the Basque Country as something suspect. But Sinn Fein and the republican movement showed us that negotiation did not have to lead to political treachery. If it could happen in Ireland, why not in the Basque Country?"

In last Sunday's elections to the Basque autonomous parliament, Otegi's party (rebaptised Euskal Herritarrok for legal reasons) got 18 per cent of the poll, 60,000 more votes and three more seats than in 1994. It was a remarkable endorsement of a switch from conflict to dialogue.

For most of the intervening period ETA and Herri Batasuna had pursued a policy of total confrontation, not only with the Spanish government, but with moderate Basque nationalist parties. ETA's killing of a young local councillor, Miguel Angel Blanco, in 1997 saw the group and its supporters face massive political and social rejection in their own heartland.

Otegi has a complex and deeply contentious explanation for how they got up and running again. Briefly, it goes like this: the Spanish government tried to manipulate the public anger about Blanco's death into a campaign against Basque nationalism in all its forms. Moderate Basque nationalists sensed an all-out attack on their culture and their country. They closed ranks with the radical nationalists behind closed doors, and elaborated a joint demand for "a say and right to decide" on whether the Basque Country would remain linked to Spain.

This proposal was launched on an unsuspecting world in September, and ETA's ceasefire followed within days. Otegi himself belonged to a branch of ETA which dissolved itself in the early 1980s to go political. At that time he was part of the minority who continued the "armed struggle", when most Basques had accepted the constitutional arrangements offered by the new Spanish democracy.

How can the hundreds of deaths that have resulted be justified? "Without ETA's struggle we would not be where we are today. There would be no possibility of independence." The fact that the moderate Basque nationalists have recently shifted from accepting the autonomous institutions "imposed on us by the Spanish state" to demanding self-determination was the key to the new process, he says.

Those who remember the maximalist demands made by ETA in the past find this hard to swallow. Surely a major reason for the ceasefire was that ETA was on the ropes, politically ostracised, and militarily debilitated after a succession of police successes in recent years?

"We consider that a frivolous suggestion," says Otegi, without raising his voice. "How can you know how strong a clandestine organisation is? If ETA had only three people and 300 kilos of Semtex, they could blow up a bus full of army officers in Madrid, and seem stronger than ever."

He casts some doubt on the translation of last Saturday's BBC interview, which suggested that "this generation of ETA" would not pick up the gun again. "ETA hasn't gone away," he says, echoing Gerry Adams. "If the Spanish government is sufficiently intelligent to say that we Basques can decide our own future, that will definitively stabilise the peace process." Otegi thinks that the Spanish Prime Minister, Mr Jose Maria Aznar, has a dilemma: "He doesn't want to go down in history as the man who missed this opportunity for peace. But he doesn't want to be known as the man who dismembered Spain either.

"The reasonable solution, from his point of view, would be to open the door to self-determination, and then to persuade the Basques that they are better off in Spain. That's the game of reason, and we are willing to play it with him. We are convinced we will win, but we are prepared to lose."