Six members of the Spanish paramilitary police force, the Civil Guard, were killed in an ambush today near the Basque city of Bilbao, apparently part of an increasing wave of violence that is made all the more menacing by reports of dissatisfaction among key Army officers.

The dead officers reportedly were killed when terrorists, presumably memebers of the Basque separatist organization ETA, attacked a small munitions convoy on a coastal road outside Bilbao. It was the most serious attack on security forces this year and brought the number of dead from political violence in the Basque region this year to 19 -- the heavist toll since ETA began operating more than a decade ago.

ETA appears to have increased its terrorist campaign to disrupt local Basque elections set for March 9. The group demands independence for Spain's Basque provinces but rejects the limited self-government that was negotiated with Spanish officials by moderate Basques and endorsed in a referendum in October.

Today's attack fuelled fears in government circles of more violence between rightists and Basques following the pattern of recent weeks. A newly formed rightist terror group in the Basque region known as the Spanish Armed Groups has claimed responsibility for five deaths this year and vowed to continue reprisal murders whenever police officers are shot.

It also raised equally serious fears about repercussions in the armed forces in light of reports of coup plots by officers who believe the government is incapable of stemming the growth of civil unrest or of guaranteeing Spanish unity.

The newspaper Diario 16 last week linked the recent ouster of the commander in chief of a tank division of the Army to meetings among his officers in which direct military pressure on the government was openly discussed.

The editor of the newspaper was charged with insulting the armed forces -- a charge carrying a six-year-maximum prison term -- illustrating government sensitivity to probes into military attitudes.

The report was denounced by the government as "unfounded and irresponsible," but well informed sources have substantiated the report.

The Defense Ministry said the removal of Gov. Luis Torres Rojas from his command was routine. He was reassigned to the northwestern province of La Coruna as military governor -- a post that does not have command of troops.

But a ministry statement admitted that another officer, Capt. Jose Tormo Rico, had been arrested for spreading "false rumors" about coup attempts.

Before taking command of the tank brigade last summer, however, Torres was in charge of a parachute brigade that has also reportedly been involved in coup plots. Known for his rightist views and his belief that the Army could end the civil unrest, the general openly talked about vague plans of action to officers who expressed their misgivings to him, sources said.

The parachute brigade was the major center of discontent, informed observers said. A Madrid magazine, Sabado Grafico, in a detailed report this week, said that officers of the parachute brigade had planned a minicoup in October to pressure the government into calling off the referendum over Basque self-government.

Confirming the report, the sources said the coup attempt centered on a routine exercise scheduled for Oct. 21, four days before the referendum.

They said that a lieutenant colonel and several middle ranking officers had demanded that they be allowed to go on the maneuvers with full equipment and munitions and that "a show of force" against the government was openly discussed in the mess hall. The general staff, however, was informed, and the plot failed. The planned exercise took place but without full equipment and under close monitoring by the higher command.

Discontent in the parachute brigade is significant, one source said, because its officers enjoy great esteem in the armed services and because the units are highly professional and do not normally contemplate meddling in politics.

Although not directly connected, meetings were taking place at the same time among officers in Gen. Torres' tank division, which is spread out in camps on the main western and northern approaches to Madrid.

Between Dec. 10 and 12, one source reported, officers met at one regiment's headquarters, where again a lieutenant colonel chaired a meeting of majors and captains to discuss how the Army could step into the politics.

The sources stressed that outside the foiled parachutist plan to carry out the October maneuvers, none of the discussions led to concrete coup plans. The main obstacle remained always that while officers detested moves toward regional autonomy, which they viewed as splitting Spain, and believed that the country was sliding into anarchy and civil disorder, they had no alternatives.

The meetings, however, have shaken the government into taking dramatic steps such as the rapid removal of Torres from his command. Also, unreported in the Spanish press, the captain general of the Madrid military region, who acts as supreme commander for the area, Lt. Gen. Guillermo Quintana, held a four-hour meeting Tuesday with officers of the tank division at the headquarters where the meetings took place.

Quintana is known for his loyalty to the government as is the head of the army general staff, Lt. Gen. Jose Gabeiras who toured Spanish north African enclaves this week. Gabeiras bluntly told a garrison meeting yesterday, "The duty of the Army is to obey the government in whom the king has placed his confidence and the executive power in which the popular will is represented."

The murder of the six Civil Guards today and the feared rightist reprisals can only cause intense unease in political circles. While the government has to show it is winning the war against terrorism, which it is far from doing, it has also to demonstrate that it can control unrest in the armed forces.

In the last analysis what makes terrorism and the breakdown of order an extremely serious cause for concern in Spain is not so much the terrorism itself -- other countries are plagued by it in Europe -- but the effect it has on the armed forces who have been traditional arbiters in Spanish politics for the past 150 years.