A secret Pakistani army document reveals that Gen. Mohammed Zia ul-Haq, who seized power in a coup last week, expressed strong opposition to taking the drastic step.

According to a highly informed Pakistani source, Zia was induced to stage the coup by six of the army's corps commanders serving under him.

They persuaded him that the take-over was essential for the good of the Army, which had come under severe public criticism during the last four months of unrest, the source said.

The document makes it clear that Zia opposed involving the army in political battling between fromer Prime Minister Zulfiqar Ali Bhutto and the opposition's nine-party Pakistan National Alliance.

Zia expressed worry that public pressure on the army following the deployment of troops to kill unarmed civilian demonstrators could lead to a mutiny or civil war. He was apparently torn, however, by what he viewed as his military duty to support the government.

In the end, though, Zia was evidently won over by the corps commanders' arguments.

The statements of Zia's opposition to a coup is in a document written May 7 and obtained today by The Washington Post, along with the two secret letters.

In it, Zia said: "We soldiers must stay above politics and let the political problems be solved by political and constitutional means."

The document, labeled "restrictive," the lowest military secrecy category, was circulated among senior officers down to the regimental level. The four-page paper, carrying Zia's signature, is entitled "Command Communication by the Chief of the Army Staff."

It was circulated two weeks after Bhutto had imposed martial law in the major cities Karachi, Lahore, and Hyderabad April 21. When troops began firing on demonstrators, a public outcry swiftly followed. It was particularly strong in Lahore, the capital of populous Punjab Province, the home of about 85 per cent of the armed forces.

On April 27, one week after martial law was imposed, Zia and the joint chiefs of staff had issued a public statement in which they said.

"We wish to make it absolutely clear that the Pakistan army, navy and air force are totally united to discharge their constitutional obligations in support of the present legally constituted government."

In his "communication" of May 7 Zia told his officers, "I expect you all to follow and implement that statement in letter and spirit; shun involvement in any non-military affairs; avoid getting entangled in any controversies and stay clearly away form active politics. The good of the army and the good of the country demands this from us."

The reason for Zia's apparent loyalty to Bhutto seems simple. In 1976, when Bhutto promoted him to chief of staff over the heads of a number of more senior general staff officers, Zia was the most junior lieutenant general in the Pakistan army.

According to a highly informed source with access to army intelligence officers, the power behind Zia - the men who pressed him into staging the coup against Bhutto - are six of the leading corps commanders: Lt. Gen. Mohammed Iqbal, Lt. Gen. Arabab Jehanzeb, Lt. Gen. Sarwar Khan, Lt. Gen. F. A. Chisti, Lt. Gen. Ghulam Hassan and Maj. Gen. Ghulam Mohammed.

Of these six, Iqbal and Jehanzeb are acknowledged to be the most politically astute and influential.

"Zia didn't want to stage the coup," the source said. "He doesn't have that kind of mentality or ambition. He was pressureed into it by the corps commanders. And they're still calling the shots."

A highly regarded political scientist said that despite their differences of opinion on the coup. Zia and the corps commanders agreed from the outset that there was a danger of mutiny in army ranks if troops were sent back into the streets to battle civilian demonstrators should the negotiations between Bhutto and the opposition fail.

In his May 7 "communication," Zia referred repeatedly to this concern.

"The leadership and the rank and the file of the defense services in general and the army in particular have been subjected to uncalled-for pressures and criticism with a view to causing doubts in their minds and trying to subvert them," he said.

"In the prevailing fog, it becomes difficult to identify truth from half-truth and reality from rumors. The simple mind tends to see things in black and white and draw conclusions based on faulty or inadequate information."

He added that he had "no doubt that the army will once again stand the test of time and fulfill its constitutional obligations to safeguard (the) well-being and solidarity of Pakistan."

Zia's support apparently buoyed Bhutto's sinking morals and convinced him that the armed forces stood with him. In a letter written to Zia on May 4, Bhutto said, "I wish to express my deep appreciation for the loyalty, sense of devotion and the patriotism you have shown in issuing the joint statement pledging the total commitment of the armed forces to safeguarding the country's integrity against external aggression as well as internal subversion."

Bhutto added that he had "taken a number of initiatives to resolve the presen political problem facing the country. I have no doubt that, with determination and understanding, all the issues can be solved in conformity with the constitutional process. It is only a matter of time."

On May 7, Zia forwarded copies of Bhutto's letter to the corps commanders. In a covering letter, Zia told th officers, "I am hopeful that we will overcome the current crisis soon. And without being modest about the whole thing. I honestly feel that the entire credit of (the) army's achievements under the current situation goes to you, your commanders, junior commissioned officers and men.

"I am confident that we will inshaallah (God willing) come out of the present crisis, not only cleanly but successfully."

In the restricted "communication" issued on the same day, Zia wrote: "What is our duty? We know that we ought to obey the legally constituted government. It is argued that the election of 7 March '77 was unfair.

"Are we in the Army justified to pass a judgement? Is there not a legally constituted machinery to adjudicate such issues? Are there not the high courts and the Supreme court to judge such allegations?

Referring to the 1971 war between Pakistan and India, in which East Pakistan became the independent nation of Bangladesh. Zia said Pakistan could not afford the "luxury" of a civil war "When our enemies are sitting across the borders waiting for such a chance. Are we prepared to offer them again an opportunity such (as) that (which) dismembered our nation only six years ago?"

"These questions must be agitating your minds," Zia continued. "The answer is quite simple and easy. We soldiers must stay above politics and let the political problem be solved by political and constitutional means.

Thus, just two months, almost to the day, before he seized power and announced the end of the Bhutto government, Zia told his subordinates that the army had no business involving itself in politics, but must obey orders.

Let (the) army not be judge regarding the legality of the government," he wrote. "The army has to obey the uninimoulsy accepted constitution of the country. There are many legal institutions to decide these issues. Let these issues be decided by those who are empowered by the onstitution to do so."