Spain's Interior Ministry said the detentions broke up one of ETA's most dangerous units, the Donosti Commando.

It is the second blow to ETA in as many days. On Tuesday, the organisation's suspected military leader, Jose Javier Arizcuren-Ruiz, was arrested in Paris.

Daniel Schweimler: Heavy blow to ETA

The nine were detained during an overnight operation in the Basque towns of San Sebastian, Hernani, Renteria, and Itsasondo.

They included two known ETA activists - Sergio Polo Escobes and Kepa Etxeberrua Sagarzazu - and seven suspected sympathisers. One report said the two known guerrillas had exchanged gunfire with police.

BBC Madrid Correspondent Daniel Schweimler says it is difficult to see how ETA can recover from the double blow.

Joint operation

The Spanish authorities have hinted that the latest arrests followed information obtained in the Paris operation.

Tom Butler: A success for Franco-Spanish co-operation

Mr Arizcuren-Ruiz, also known as Kantauri, was arrested at the Hotel Printania with three other suspected ETA members.

He was detained by French police in a joint operation with the Spanish civil guard. Two further ETA suspects were arrested at another location. All had been armed.

Paris protest

The Basque nationalist party Herri Batasuna has accused France and Spain of trying to destroy the peace process between the separatists and the Spanish Government.

Four ETA suspects were captured at this hotel in Paris

Following Kantauri's arrest, Herri Batasuna spokesman Arnaldo Otegi said that Tuesday's action showed France and Spain had adopted a strategy of war.

"It shows the degree of political irresponsibility ... and makes clear their choice - an alternative of war, war and more war," he said.

Inaki Anasagasti, spokesman for the mainstream Basque Nationalist Party, which helped convince ETA to declare a ceasefire in 1998, said the arrests would "not help" the peace process.

Peace pledge

But Spanish Interior Minister Jaime Mayor Oreja said his government remained committed to finding a peaceful solution to the conflict in the northern Basque region.

He said: "Some may ask whether detaining these people brings us closer to peace or further from it. I say keeping within the law of the state always ensures we are closer to peace."

Herri Batasuna is organising a protest later this month and has called on its supporters to think about how to respond to what it called "this attack and the attacks that will come in the future".

Two journalists accused of collaborating with ETA were arrested in the Basque region earlier in the week.

Ceasefire

ETA declared a ceasefire last September after a 30-year campaign of violence for an independent Basque state. However there has been little movement in establishing a peace process since the ceasefire was declared.

There are almost daily attacks in the Basque region on property belonging to local politicians, and on what are seen as symbols of the Spanish state, such as post office vehicles and banks.

The Interior Ministry said the Donosti Commando had been responsible for the killing of six town councillors of the ruling Popular Party, including Miguel Angel Blanco whose kidnapping and execution in 1997 sparked nationwide demonstrations against ETA.