MADRID — The suspected leader of Eta, the Basque separatist group, has been arrested, striking a blow to the terrorist organization responsible for a series of recent attacks in northern Spain.

French and Spanish police seized Francisco Javier Lopez Pena and three other members of the group in Bordeaux late Tuesday night.

The Spanish prime minister, Jose Luis Rodriguez Zapatero, said: “This is a major police operation which has dealt a very strong blow to the leadership of the Eta terrorist group.

“There is no doubt that they had great responsibilities within the Eta terrorist group.” He added that the arrests signalled “another step towards the victory of democracy over terrorism.”

He used the latest development as an opportunity to stress the government’s commitment to fighting terrorism.

Mr. Pena, who has used the alias Thierry and been on the run since 1983, is suspected of being the mastermind behind a series of recent attacks, beginning with the car bomb at Madrid international airport in December 2006, which killed two people and brought an abrupt end to a fledgling peace process.

He is believed to have taken over Eta’s underground leadership in 2006 when the group was holding peace talks with the government of Mr. Zapatero.

According to Spanish media, Mr. Pena participated in the talks but then decided to end the ceasefire.

He is mentioned in a court order related to the arrest of Iker Aguirre, a suspect in the Madrid airport bombing, according to the Web site of El Pais. The court order says Pena taught Aguirre how to manage and build explosives.

In the raid on the Bordeaux apartment, police discovered hand guns and seized at least one computer.