Islamabad, Pakistan — General Zia's office is surprisingly Spartan. Other than portraits of Pakistan's founders - Muhammad Ali Jinnah and Muhammad Iqbal - nothing hangs on the walls. In an otherwise empty bookcase are an alarm clock and two gold-plated ''ayats'' (quotations) from the Koran. The office is located off an ornate Roman-Mogul hall, almost, one senses, there as an afterthought.
Then Gen. Muhammad Zia ul-Haq steps into focus, wearing the uniform of a four-star general, epaulets of the Army chief of staff. His bearing is rigidly military.
He explains that the office - in a complex of splendid, imperial buildings inherited from the British Raj - is as temporary as he is at the helm of his nation's affairs.
He insists to a visitor that he considers his presidency ''interim.'' Not only has he left the lavish presidential office suite unoccupied, but he has also never seen the presidential living quarters in Pakistan's capital, Islamabad.
He implies strongly that he will form an Islamic party and will go to the people to get their mandate. He seems almost embarrassed, an apologetic leader of a martial-law regime.
There are those in Islamabad who will tell you that General Zia, as Army chief of staff, was propelled into seizing power in 1977 by a forceful coupling of disgruntled military and civilian bureaucrats. According to the theory, as street rioting gripped the country, he was told without fanfare: ''Either (then prime minister) Bhutto goes or you both go.''
Yet on July 5 the man who initially seemed a rather jolly general, unambitious and sycophantic to the few who knew him well, completes his sixth year in power. He chuckles with relish at the pundits who insisted he would not be around for long.
But will the often contradictory Zia throw his hat into the political ring? Pakistani officials are not as sanguine as they were earlier this year.
One reason is that Zia's dreams of Islamic harmony appeared to go up in smoke as the Sunnis and Shiites of Karachi battled each other in three months of sectarian street violence. Though he may have pushed through his controversial program of ''Islamization'' for largely political ends - something alleged by his critics - few question his belief in Islam and that Islam is the key to understanding the man. Militant men of religion
Thus, as Pakistan's increasingly militant men of religion preach violence from the mosque, there are those who believe that the smoldering barricades of Karachi may have derailed the immediate political ambitions of General Zia.
Zia is as enigmatic as the print of the ''Mona Lisa'' that hangs in his sitting room. He continues to perplex Pakistani political pundits and Western diplomats. Obsessively guarding his privacy, never fashioning a personality cult , he projects a contradictory mosaic of Muslim, general, and man.
Is he modest or Machiavellian? Superficial or shrewd?
Whatever the answer, the general is not a transitory leader.
His modesty seems compelling, yet, beneath it, one often glimpses a hard, flinty edge. As a brigadier general in 1969, he arrived for a formal wedding in the city of Sialkot, squeezed among peasants, aboard the local bus. Far more junior officers arrived in sleek, official cars.
''A peculiar chap,'' said a guest at the wedding. ''He could certainly have come in a staff car. . . .''
Imbued with the spirit of the British Indian Army, in which he served for three years, he still refers to the ''old chappies'' with affection and is loyal to a fault - once he has made a friendship - according to trusted friends. Yet Bhutto promoted him over the heads of seven generals to become his Army chief of staff. Then Zia deposed the flamboyant prime minister and, despite world appeals for clemency, he allowed Bhutto to hang.
General Zia is a devoted father, who particularly dotes on his 11-year-old handicapped daughter. She pops in and out of official meetings, interrupting visiting presidents at will. He has valued education for all of his children, including his two elder daughters. One is a medical student, the other a computer analyst. Yet, he has confounded even some supporters by permitting the most reactionary of Pakistan's religious men to draft a series of Draconian laws that, if enacted, would take women back to the days preceding the Koran.
When he talks about himself - a rare occurrence - the general provides a visitor with insights into the loneliness of power. (He is said to work 18-hour days.) Then, in nearly the same breath, he disarms you, making it patently clear that he absolutely relishes traveling in the company of kings and presidents.
If there is one attribute of the martial-law leader on which most Pakistanis agree, it is that he is adept at dividing and ruling. For, beneath the humble demeanor, the old world charm, Zia ul-Haq is first and foremost a survivor in the classical sense. Self-preservation is in no way contrary to the precepts of the Koran.
Born 23 years before his country was founded, in the city of Jullundur in what is now the Indian state of Punjab, he had an early, all-absorbing commitment to an independent Pakistan. His father, Akbar Ali, a middle-level civil servant in the Raj, was also a ''molvis'' - or devout religious man - and, according to the Pakistani leader, a strict disciplinarian. The young Zia and his six brothers and sisters were awakened at dawn to begin the ritual of six daily prayers. ''Long before I understood it,'' he says, ''I began reading the Koran.''
Within the modest Jullundur household, one never questioned the more orthodox interpretations of the Koran.
Young Zia spent summers in the hill station of Simla, as his father followed the British administration north. He was educated at New Delhi's prestigious Saint Stephen's College, an Anglican missionary school. And he completed two military training courses in the United States. These experiences left an impression that come to the surface in expressions such as ''old chaps'' and ''by jingo'' that still creep into his speech.
British and American officers made him obsessed with the need to keep physically fit. Even today, he sometimes rides a bicycle between office and home , despite a phalanx of black-bereted bicycling security guards. The road to Pakistan
As a member of the British Indian Army and a newly commissioned lieutenant in the occupation-liberation forces that went into Burma, Malaysia, and Indonesia in 1945, Zia saw few of the horrors of war. That came later, in November 1947, when British India was partitioned and Pakistan was born.
Then a captain, his family in a refugee camp, Zia was escort officer for the last train to leave Babina, an armored corps training center in Uttar Pradesh. He took it to New Delhi and then in west Pakistan.
''It was ghastly,'' he said in an interview. ''It continues to haunt me even now. It should have been a 16- to 18-hour journey. It took me seven days. I had consignments of military equipment. Some 50 refugees. We were under constant fire, the country was burning, until we reached Lahore. . . . Life had become so cheap between Hindu and Muslim. It was a civil war.''
''I saw very little greenery. Little evidence of life. Only the mutilated bodies of men and women lying along the rail line. You can't imagine my feeling when I finally reached Lahore. I felt, for the first time, that I could smell free air. . . . This was Pakistan.''
It was 35 years before Zia returned to India - in November last year, carrying with him a draft proposal for a no-war pact. His Army, in the years since independence, had gone down to three ignoble, Indian defeats. His and India's is a competitive arms buildup, a competitive nuclear race. The two nations hold some of the world's deadliest, most sophisticated arms. The stakes are now considerably higher than they were 35 years ago.
Nearly six years in power had transformed Zia from soldier to politician, albeit still under a regime of martial law. The Soviet invasion of Afghanistan, his neighbor to the north and west, had made him a formidable figure in the eyes of the Western world. And, in Islamic circles, he was a Sunni general facing down the Shiite Ayatollah, exporting revolution from Tehran.
For General Zia his journey to New Delhi last November was a sentimental one. He paid a visit to his alma mater, Saint Stephen's College.
''Yes,'' he said, ''I'm a product of a Christain missionary school, with all the zeal of a Muslim. . . . It was there that I first joined a small, dedicated cadre of the Muslim League. We didn't burn buses, or create turmoil, but, in our own quiet way, we were part of the independence movement. In 1943, I even went to the extent of selling copies of Dawn [an underground Muslim paper]. And, by jingo, that was considered quite revolutionary in my day.''
Some years ago, Zia ul-Haq reportedly told a group of visiting American senators, ''I've discovered that gaining power is much easier than giving it up.''