September events

1st: ETA admits killing four people in August 8th: Spain approves draft anti-terrorist laws 13th: Spanish police arrest "ETA's political leadership" 14th: Gun attack on former Socialist Basque politician linked to ETA

Mr Arregui, 45, is accused of being behind a failed assassination attempt against the Spanish King, Juan Carlos, five years ago.

The arrest comes two days after Spanish police raids detained 20 people, dismantling what they said was almost the entire political leadership of ETA.

Increasingly active

Mr Arregui, who is also known as Inaki of Renteria, is suspected to be the man in charge of the political and military wings of ETA.

But the BBC's correspondent in Barcelona, Flora Botsford, says if Mr Arregui is the ETA chief, the arrest will make little difference to the group's campaign in the short term.

We have to persevere in the anti-terrorist struggle

Spanish interior minister

She says ETA is run by a number of small cells, rather than a single person.

Spanish Interior Minister Jaime Mayor Oreja said the latest arrests proved that Spanish police, in co-operation with the French authorities, were capable of striking at the heart of ETA.

"I know that all of Spanish society has been through bad times in the past few months and that on occasion there has been a loss of confidence," he said. "But we have to persevere in the anti-terrorist struggle."

Nicolas Redondo Terreros, head of the Basque branch of the Spanish Socialist Party, said he hoped Mr Arregui's arrest meant that police had "chopped off the head" of the separatist group.

ETA, which has been fighting for an independent homeland for 30 years, has been increasingly active in recent months.

International warrants

ETA has been linked to 12 killings in Spain since calling off a 14-month ceasefire last December.

Most of the group's leaders are believed to have been hiding in southern France since a crackdown by the police in Spain's Basque region.

Former Basque separatist spokeswoman Ana Lizarralde was detained on Wednesday

Mr Arregui has had four international arrest warrants issued against him by Spanish judges between 1987 and 1996.

He is believed to have taken control of ETA in 1992 following the arrest, also in Bidart, of the man thought to have been leading the movement at that time, Francisco Mujica Garmendia.

Born in Renteria in the northern Spanish province of Guipuzcoa, Mr Arregui's association with ETA goes back to the mid-1970s. At that time he was a member of ETA's special attack units.

On Wednesday, about 300 Spanish police officers were involved in dawn raids in the northern Basque region, the neighbouring region of Navarra and in Madrid.

The operation targeted what the government described as a clandestine financing group for ETA.

The group, EKIN, strenuously denies it is part of the ETA command structure and describes itself as a grassroots co-ordinator of Basque pro-independence groups.

On Thursday, a former Socialist party official was shot and wounded in the Basque city of San Sebastian, hours after Spain's interior minister warned of a backlash following Wednesday's arrests.