Madrid  - Spanish Interior Minister Alfredo Perez Rubalcaba on Wednesday stressed the importance of the capture of Francisco Javier Lopez Pena, the political leader of the militant Basque separatist group ETA, and three other ETA suspects in France overnight.

"It is not just another operation," Rubalcaba said in the Senegalese capital Dakar. He described Lopez Pena alias Thierry, 49, as the person with "the most political and military weight" within ETA.

Spanish police Wednesday detained a former Basque mayor on charges of having met with the four suspects.

Jose Antonio Barandiaran Ezama was held in the Basque town of Andoain, where he was mayor from 1999 to 2003 for the Basque separatist party Euskal Herritarrok (EH), currently banned under the name of Batasuna.

Rubalcaba said that a sixth person had also been detained in France in connection with the flat where the suspects were captured in the south-western city of Bordeaux.

Lopez Pena and the three others were detained in a flat in the city centre where they were holding a meeting, reportedly to plan attacks.

The four carried pistols, but did not resist arrest. Police found a small quantity of explosives as well as documents and computer materials in the flat, which was to be searched more thoroughly on Wednesday.

The arrest of ETA hardliner Lopez Pena was regarded as the heaviest blow to the group in years.

Lopez Pena is held partly responsible for the failure of Prime Minister Jose Luis Rodriguez Zapatero's attempt at a peace process with ETA in 2006, and is believed to have ordered the group's main attacks since then.

Lopez Pena is believed to have joined ETA in 1980. After serving a prison sentence for extortion, he became responsible for ETA's arms caches, trained terrorists, and rose to become one of ETA's top leaders in the recent years.

In the summer of 2006, the Zapatero government announced a peace process with ETA in an attempt to end the group's armed campaign that has cost more than 820 lives over four decades.

Towards the end of the process, Lopez Pena replaced another ETA leader, Josu Ternera, in meetings with the government, tabling tough demands on Basque self-determination that the government had not expected, according to media reports.

Lopez Pena then allegedly ordered a car bombing that killed two Ecuadorian immigrants in a Madrid airport underground parking lot in December 2006. The attempt to pressure the government caused the peace process to definitively collapse.

Lopez Pena ordered an official end to ETA's 14-month ceasefire in June 2007, and launched a violent campaign to make the government "pay" for the failure of the peace process.

Attacks believed to have been ordered by Lopez Pena included the killing of former Socialist councillor Isaias Carrasco two days before the March 9 general elections, and a car bombing that killed police officer Juan Manuel Pinuel a week ago.

Lopez Pena was the third party in ETA's current leadership, which also includes two military leaders, according to the daily El Pais.

In addition to living in France, where ETA has long had a large part of its infrastructure, Lopez Pena has spent time in Cuba to stay out of the reach of Spanish police.

The ETA members who were captured with him were Ainhoa Ozaeta Mendiondo, who was Barandiaran Ezama's deputy mayor and announced the beginning of the ceasefire in March 2006; Igor Suberbiola, suspected of planning attacks against Spain's tourism branch, and Jon Salaberria.

Salaberria is a former member of the Basque regional parliament for Batasuna, the political wing of ETA which was outlawed in 2003. He is suspected of helping to finance ETA through a network of separatist bars in the Basque region.

The arrests followed a months-long joint French-Spanish investigation, which gathered speed after the car bombing that killed Pinuel in the Basque town of Legutiano.

The Spanish government has cracked down on ETA since the end of the peace process. Dozens of suspected ETA members or collaborators have been detained.

ETA leaders have been captured some half a dozen times since 1986. The arrest of Lopez Pena was regarded as the most important one since that of political leader Mikel Albisu in south-western France in 2004.

In its most recent attack on Monday, ETA bombed a nautical club in the Basque town of Getxo, causing serious material damage but no injuries.

On Tuesday, Zapatero rejected a request by the Basque regional government for a regional referendum on self-determination. Regional Prime Minister Juan Jose Ibarretxe believes the vote would help to solve the problem of ETA. (dpa)