The two main parties have been at loggerheads since the socialists were unexpectedly elected to power days after the train bombings in March 2004, which the right-wing Popular Party (PP) first blamed on Eta despite evidence of an Islamist connection.
After the meeting on Tuesday, Mariano Rajoy, the PP leader, said: "I have offered the prime minister my support and that of my party to deal with Eta, putting aside our ideological and party differences."
Jose Luis Rodriguez Zapatero, the prime minister, said he had regained confidence in Rajoy but said that after two years of virulent party politics, it would take some time for the two sides to trust each other fully.
The two leaders, who have only met only a few times, were still cautious on Tuesday, posing for photographs before meeting but holding separate news conferences afterwards.
Rajoy, whose party has taken a very hard line against Eta, said the group should hand in its weapons and urged the government not to make any concessions to the separatists.
Rajoy insists that Eta disarm
Zapatero said: "The government is already working to make sure that this is a permanent ceasefire, that is the beginning of a definitive, absolute, total end to all types of violence."
Last week, Eta declared a permanent ceasefire, raising hopes that its violent 38-year campaign for an independent Basque state straddling the French-Spanish border was at an end.
Eta, which has been blamed for killing 850 people over the years, did not repeat its traditional call for "self-determination" in its truce statement but instead called for a "democratic process" to decide the Basque country's future.
Juan Jose Ibarretxe, the Basque government leader, has called all parties in the region together this week to discuss the post-truce situation, including Batasuna, the party outlawed for its links to Eta.
However, a Basque government spokeswoman said the Basque wing of the PP had declined the invitation.