MADRID—Spanish Prime Minister José Luis Rodriguez Zapatero is under fire from political opponents over what they say is his government's lenient approach to Venezuela after allegations of links between Caracas and Spanish terror group ETA.

Last week, a Spanish judge ordered the detention of 12 members of ETA, a Basque separatist group, and Colombia's main guerrilla group, known as the FARC, for planning to assassinate Colombian political figures, including President Álvaro Uribe, during visits to Spain.

The same day, Mr. Zapatero asked Venezuela to "explain" the allegations of cooperation with the ETA and FARC made by Spanish Judge Eloy Velasco. Venezuelan President Hugo Chávez, who denied any ties to the FARC or ETA, blasted Spain as a nation that still considered itself Venezuela's colonial master.

Venezuelan Foreign Minister Nicolas Maduro dismissed the court opinion.

On Wednesday, Mariano Rajoy, the head of the Popular Party, Spain's largest opposition party, said Spain had been craven in the face of insults from Messrs. Chávez and Maduro.

Mr. Rajoy said Spain should issue an official protest against Venezuela's criticism of the Spanish judiciary. "If not, we will leave the sensation that Spain or its government can be insulted without paying a price," he said in Spain's Parliament, to applause from this partisans.

The spat with Venezuela has put Mr. Zapatero—who has worked to keep good relations with the volatile Mr. Chávez—in a difficult position. In Spain, there is scant support for ETA, which in its quest for an independent Basque state, has killed more than 800 people in bombings and assassinations during the past five decades.

But Venezuela is an important trading partner for Madrid. For instance, the Spanish oil firm Repsol SA recently headed a consortium of firms that won one of two projects awarded in the first oil auction during Mr. Chávez's 11 years in power. Some 300,000 Spaniards, including Venezuelan-born sons of Spanish immigrants who have obtained Spanish passports in recent years, live in Venezuela. After Argentina, the country is home to the second-largest Spanish expatriate commmunity—most opponents of Mr. Chávez.

To read full Wall Street Journal story — Go Here Now.