(Adds nationalist rally, paragraphs 7-12)

MADRID, March 31 (Reuters) - Police found 140 kilos (310 lb) of explosives on Saturday in the Basque Country and neighbouring Navarra region in northern Spain, the second such haul in four days, officials said.

Basque police are on maximum alert following the arrest of eight suspected members of the armed separatist group ETA and the discovery of explosives and other bomb-making equipment on Wednesday.

Saturday's stash came to light during six morning raids by police, the Interior Ministry said in a statement.

Police believe those arrested this week could be responsible for 24 ETA attacks between 2004 and 2006, Interior Minister Alfredo Perez Rubalcaba said in comments broadcast on state radio.

ETA has waged a campaign for independence for the Basque Country over 40 years, killing about 800 people.

Spain's government is working to end the violence, but it froze contacts with ETA in December after the group broke a nine-month ceasefire by blowing up a Madrid airport car park and killing two people.

A rally of Basque nationalists passed without incident on Saturday in a conference centre in the Basque town of Barakaldo, police said.

Organisers had planned the rally to launch a new political party, but on Friday the Spanish High Court banned the gathering, saying the new party was a cover for the outlawed separatist party Batasuna.

The government has banned Batasuna for links to ETA and has said it can only participate in elections once it renounces violence.

Basque nationalists want to launch the new party -- Abertzale Sozialisten Batasuna -- to seek a way around Batasuna's ban in time for May regional elections.

The court on Saturday allowed the rally to go ahead, provided no mention was made of the new party. More than 15,000 people attended the meeting, state radio said, including Batasuna leader Arnaldo Otegi.

Police could not confirm how many people had attended but said the rally had obeyed the court by not mentioning or displaying the new party's name.

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