Nine Civil Guard police cadets died and more than 40 persons were injured today when a car bomb blew up a bus carrying the guardsmen. Authorities blamed the Basque separatist organization ETA for the attack.

The incident was viewed by officials both as an attempted show of strength by the terrorist group on the eve of formation of Spain's newly elected parliament and as retaliation for France's deportation over the weekend of an alleged ETA leader to West Africa.

Although no one has asserted responsibility for the attack, police said they had no doubts that ETA was responsible. The explosion was similar to previous ETA attacks on buses carrying policemen, including one that killed five in April and another that killed an American bystander and injured 20 policemen last September.

After today's attack on the bus, taking young cadets of the Civil Guard's traffic school to the city's outskirts for a training program, injured and dead lay scattered over a wide area. Cars and houses in the vicinity were severely damaged, and a huge pall of smoke hovered over the scene of the attack, which took place shortly before 8 a.m. in a quiet residential area.

Forty of the injured -- including several civilian bystanders -- were hospitalized, several of them critically wounded. Eight of the guardsmen were killed instantly; they were in their late teens and early 20s. A ninth died later in a hospital.

The bomb, hidden in a parked van, was detonated as the police bus passed. Police estimated that the bus was hit by 100 pounds of explosives mixed with nuts, bolts and bicycle chains packed into the bomb as shrapnel.

Last September, ETA used the car bomb method for the first time in Madrid, and the blast injured 20 guardsmen, who were in a bus being taken to embassies for security duty. A bystander, Ken Brown, an American in Madrid on business, was killed in that blast.

The bomb explosion today appeared likely to harden opinions against ETA and to prompt calls for increased police measures in the Basque country. The conservative opposition called for emergency powers to deal with ETA, and the ruling Socialist Party, which won the June 22 elections, appealed for increased cooperation with police to root out the terrorists.

The attack came at the start of a busy week of Spanish political activity that includes the first meeting of the new parliament.

Prime Minister Felipe Gonzalez, who is expected to be asked by King Juan Carlos to serve for a second term, is considered likely to place the eradication of ETA high on his list of priorities.

The blast also came just after a key development in the longstanding battle to contain ETA terrorism. Over the weekend, French authorities put Domingo Iturbe, 42, alleged to be ETA's leader, on a plane for Gabon, a former French colony in West Africa.

Iturbe, who since 1968 has enjoyed refugee status in southwest France despite pleas from Spain for his extradition here, was arrested by French police shortly after the April bombing here. He had been detained for violating residency requirements -- refusing to comply with an earlier confinement to northern France, well away from the Spanish border.

The decision Saturday to expel Iturbe to Africa underlined the French government's commitment to rid itself of ETA suspects who traditionally have sought haven there. In the past 2 1/2 years, 36 alleged ETA members living in what is known as the French Basque country have been deported to far-flung locations, including Ecuador, Panama, Togo and the Cape Verde Islands.

Iturbe's deportation was strongly criticized in Basque nationalist circles. The popular separatist had been considered by many to be the only ETA leader capable of negotiating an end to Basque terrorism with Spanish authorities.

Over the past 18 years, ETA -- an acronym in Basque for Basque Homeland and Liberty -- has asserted responsibility for, or been linked to, an estimated 500 killings.