As Pakistan prepared for her first direct democratic national election last December, a visitor to Dacca commented on the number of khaki-shirted soldiers discreetly sequestered along the back streets of the humid East Pakistani capital.

"Oh, they're not troops," a knowledgeable Pakistani said.

"But I saw them," the visitor persisted.

"You saw East Pakistani militia," the Pakistani explained. "The Government would never send its troops in here."

Times have changed. Eager in December to preserve the fragile authority of the Western-based national Government amid the autonomy-minded East Pakistanis, President Agha Mohammed Yahya Khan avoided antagonizing the easterners with signs of his martial-law administration.

Reports of Battle

Yesterday, federal troops were reportedly used against an East Pakistani insurrection.

Although the December election proceeded peacefully, the eruption of civil war between the East and West has been a constant specter since the two parts of Pakistan emerged from the partition of the Indian subcontinent on Aug.15, 1947.

Separated by 1,100 miles of Indian territory, West and East Pakistan are bound chiefly by their common religion -- their 130 million people constitute the world's largest Moslem state.

But in other ways the two sections are as different as the Roman Catholic countries of Spain and Poland.

The Bengalis of East Pakistan are a small, dark people who are proud of their poets and scholars. Their life in the humid Ganges-Brahmaputra delta revolves around the monsoon-nourished rice crop. Their culture is akin to that of the Bengalis of neighboring India.

The Punjabi-dominated West Pakistanis, heavier and lighter skinned, are considered to be more martial and stolid. Their culture is rather Middle Eastern and their Urdu language, a Persianized Arabic written right to left, is incomprehensible to the Bengalis, who speak their own language. The West Pakistanis eat grain and meat and generally enjoy a higher degree of industrialization than their eastern countrymen.

Despite the East's larger population--about 75 million to the West's 55 million--the West with its 310,403 square miles has always dominated the eastern wing of 55,126 square miles.

In fact, Bengali nationalists have long charged that West Pakistan has been as much a colonial exploiter of the East as was Britain a generation ago.

Figures show the price of wheat and rice is twice as high in the East as in the West, although Bengalis earn on the average only half as much as their Western countrymen. In a comparison between the two wings, six times as much electricity is produced in the West, four times as much foreign aid is spent there, three times as many imports are consumed there, twice as much development money is allocated there and nine times as much in defense funds are spent there.

Despite the larger population of the East--if the United States had the same density it would have 4.5 billion people--the Bengalis have 13 per cent of the electrical power, 25 per cent of the telephones, 20 per cent of the motor vehicles and 15 per cent of the secondary school teachers in the country.

The Bengalis have bridled under such realities since the founding of the state.

The Bengalis are thus unified and fanatically determined and while federal troops may put down the insurrection militarily, widespread guerrilla war might continue.

The nightmare of Pakistan's founders seems to have become a reality, posing some big questions: What would be the position of an independent East Pakistan? Would she fall under the influence of China, India or the Soviet Union or the United States? Would a struggle by those nations for a foothold in the new state set off grave new tensions in the sensitive region?

This was probably in the mind of President Yahya Khan when he recently told a countryman who wished him a long reign: "For God's sake have mercy on me and spare me."