MADRID, OCT. 9 -- The discovery of a master file of the Basque terrorist group ETA in a French farmhouse last week has led to an unprecedented wave of high-level arrests and dealt a crippling blow to the separatist organization, Spanish officials said.

A roundup of Spanish Basque militants in southwestern France over the past week marked a sharp increase in French cooperation with Spain against the underground group -- whose initials stand for Basque Homeland and Liberty in the Basque language -- these officials said. The French government, in what is seen here as an important reversal, apparently has decided to end the French Basque country's traditional role as a haven for radicals sought by Spanish police in connection with ETA bombings and assassinations.

This is regarded by Spanish officials as an indispensable condition for effective steps against the illegal military organization, which has been blamed for more than 500 deaths in two decades of violent struggle to win self-determination for Spanish Basques and, more recently, move beyond the broad autonomy already granted the region.

More than 50 Basque militants captured in France have been turned over to Spanish authorities in the past week and another 50 have been arrested here on the basis of information supplied by French police, Spanish authorities said.

The most important of those arrested in France was Santiago Arrospide. Spanish authorities said he has been chief of operations for ETA terrorist strikes for the past 18 months. Jose Maria Rodriguez Colorado, director general of the Spanish National Police, said he was possibly ETA's top leader.

Officials in Prime Minister Felipe Gonzalez's government said they will seek Arrospide's extradition to stand trial here on terrorism-related charges.

Arrospide's capture in southwest France on Sept. 30 set in motion the dragnet for other Basque extremists. French antiterrorism police who picked him up along with several other suspects said they found a large cache of ETA documents in a house at the small village of Saint-Pee-sur-Nivelle where Arrospide was hiding.

Within several hours, Spanish authorities were notified and specialists arrived from Madrid to examine the find.

A Spanish Interior Ministry official said the specialists quickly determined that Arrospide's documents contained an extraordinary array of detailed information on ETA activities and cells in Spain, including names and addresses, as well as 150 names and locations of French Basque militants who had offered refuge to ETA members.

"He had everything written down," the official said.

Among those captured since then were Jose Ramon Gonzalez Valderrama and Miguel Minguez Garcia. Spanish authorities described Gonzalez as ETA's top expert on electronic fuses and detonators for terrorist bombs, and they said Minguez was ETA's press and propaganda chief.

Police in Barcelona, also using the information from Arrospide's files, arrested several Basque militants who, they said, were sent in by Arrospide to set up a new ETA cell there.

ETA structures in Barcelona were believed dismantled last September by police investigating the bombing of a supermarket in July in which 23 persons were killed.

"This is a very strong blow," acknowledged Carlos Garaikoetxea, head of the Basque nationalist Eusko Alkartasuna party, in a statement to Spanish reporters in Bilbao.

Basque militants staged a number of demonstrations in Bilbao and other Basque towns Wednesday to protest the French decision to turn over large numbers of arrested Basque nationalists to Spanish police.

Buses and cars were set afire in several towns, and a number of protesters were injured, according to news reports from the region.

Robert Pandraud, the French deputy interior minister for security, said in Paris that cooperation with Spanish authorities against ETA fit into a pattern of growing international cooperation against terrorism.

Interior Minister Charles Pasqua, who has made antiterrorism a pillar of Prime Minister Jacques Chirac's political platform, also has increased cooperation with the United States and other countries in tracking Middle Eastern-related terrorism.

A Spanish official said French desires to crack down on its own underground Basque group, Iparretarak, also contributed to the increase in cooperation with Spain. The recent death of a French policeman in southwestern France was attributed by Pandraud to Iparretarak militants, in what he called a "bad turn" to ETA.

French Basque nationalists traditionally have remained relatively peaceful as part of an understanding with ETA, Spanish sources said. The bargain assigned the French Basque country a role of safe haven for Spanish militants in exchange for a promise that French Basque aspirations would be addressed once the Spanish Basque country won its battle.

In addition, French governments traditionally have been reluctant to strike hard against Spanish Basques seeking refuge. France has long cherished its reputation as a country of asylum.