A package bomb explosion injured two mailroom employes yesterday in Madrid's daily newspaper El Pais, a leading advocate of freedom of expression and democratic change.

In a front-page editorial in today's edition, the newspaper blamed forces trying to destablize the country on the eve of parliament's final approval of a new democratic constitution - the first since the 1936-39 civil war.

Madrid newspaper, including El Pais, received anonymous calls blaming the blast on both the extreme leftist GRAPO underground group and the Anit-communist Apostolic Association, an extreme rightist organization that blew up the office of a leftist satirical magazine last year in Barcelona.

Spain has been subjected to killings and bombings by right-wing and left-wing extremists apparently bent on provoking military intervention to upset the country's transition from dictatorship to a democracy.

In the last month Basque separatist guerrillas have killed 13 policemen and paramiltary Civil Guards, one navy officer, a forest guard and an alleged police informer. Government and police officials had anticipated new violence as both houses of parliament scheduled a special session today for final approval of the new constitution, designed to give Spain the foundation of a Western-style democracy and prepare for new general elections and the first free municipal elections in 40 years.

The charter, which will be sumbitted to the people in a referendum, probably next month is expected to pass overwhelmingly. Only Basque nationalist deputies and senators, who have opposed provisions for self-rule, have said they will abstain.

For the extreme left and the extreme right, El Pais has become a symbol of democracy. It became a major political force and independent organ of information five months after the death of dictator Francisco Franco. Its circulation quickly soared to 150,000 and its new columns and editorials became required reading for government officials, businessmen, diplomats, intellectuals and foreign journalists.

The newspaper, a frequent critic of the military, police and rightist and leftist violence, has received hundreds of threats in the past two years. Its mail is opened in a special room on the fourth floor and employes there were expected to exercise extreme care.

A young clerk suspected the package, mailed in Madrid, contained a bomb. But his warning was ignored and the box exploded when it was opened. Apart from seriously injuring two clerks and lightly wounding the clerk, the blast caused little damage. About 100 of the newspaper's 350 employes were in the building at the time.

Immediately after the explosion, the government warned all Spanish news agencies and newspapers to be on the alert for letter and package bombs. The offices of Arriba, the government's newspaper in Madrid and an outspoken supporter of Premier Adilfo Suarez, were evacuated following a telephoned bomb threat. But nothing was found.

In recent weeks police have taken extraordinary measures to protect government officials and King Juan Carlos I. The King has been a major