The State Department decision to block a shipment of tear gas to Pakistan is reported to have cause Prime Minister Zulfigar Ali Bhutto's government to reconsider its long-standing friendship with the United States.

Political opponents of the beleagured Prime Minister have taken heart from the U.S. move, one of them terming it a "source of great strength" for the people attempting to force Bhutto out of power.

An opposition attorney, M. A. Kasuri, told reporters in Lahore, the Punjab provincial capital, that Pakistani police had used tear gas "frequently against women, children . . . and even people praying in mosques."

The Pakistan government, however, has turned to other sources for the tear gas, and, according to several Pakistani and foreign sources, a delivery is expected to arrive shortly. There has been speculation that a British company was making the sale, but this could nto be confirmed.

Bhutto's supporters, meanwhile, have begun spreading claims that the United States, particularly the Central Intelligence Agency, have funded the oppostion Pakistan National Alliance with huge payments, as high as $25 million.

In demonstrations yesterday here in the capital members of Bhutto's Pakistan People's Party marched through the streets shouting "Down with Jimmy Carter" and "Death to imperialist lackeys."

The decline in the Bhutto government's regard for the United States comes as the U.S. postion appears to be improving across the border in India. The coincidence, although government sources concede it may be nothing more than that, has been carefully noted here.

"With the advent of a new, right-wing government replacing Mrs. Gandhi in India, it is entirely possible that the United States is making some sort of deals in New Delhi at our expense," a highly placed government figure said. He refused to speculate as to what these alleged "deals" might be.

Nevertheless, Pakistani officials are seriously questioning the reliability of the United States as a friend and ally. Their doubts, which surfaced periodically in the past, have been refreshed by last week's State Department turn-about on the $68,000 tear gas shipment. Relations between the two countries began cooling last year when the United States opposed the sale of a French nuclear repotrcessing plant to Pakistan.

Until then, relations between the United States and Pakistan had been relatively goof, and mutually beneficial, for a decade. The most notable development, perhaps, was the role Pakistan played in clearing the way for former Secretary of State Henry Kissinger's first trip to China, in preparation for the visit of then President Nixon.

Bhutto, smarting under continuing popular demonstrations against him resulting from allegations of massive rigging in last month's general elections, claims to see a sinister U.S. hand at work.

Wheter he genuinely believes this is simply grasping at straws is a subject for much conjecture in government and diplomatic communities. One source pointed out that Bhutto had begun expressing anti-U.S. comments even before the tear gas decision was made.

In his inaugrual address March 28, for example, he mockingly referred to the Carter administration's concern with human rights by saying that if his party had been given $25 million - the amount allegedly passed by the CIA to the opposition - "I could afford to be worried about human rights, too."

A senior Pakistani official, who said that Bhutto "would prefer" to be on good terms with the United States for fear of Soviet intervention in Pakistan, added that the government saw the tear gas decision as an "anti-Bhutto signal.

Several Western diplomats here tended to share this assessment. "It looks as though Washington has decided that Bhutto is not long for this world, as prime minister, so why back a loser with a weapon, even an innocuous weapon like tear gas?" one said.

Another diplomat said, "As I see it the United States stood to lose no matter which way it decided."

Despite the ill will generated by the tear gas matter, U.S. supplies of material continue to arrive here, according to one U.S. source, if all contracts now under negotiation between Pakistan and American arms manufacturers are signed, the total sale of U.S. military equipment to Pakistan this year will be $150 million.

These contracts include sales of wire-guided TOW anti-tank missiles, bombs, machine-gun ammunition, communications equipment, vehicles and two naval destroyers, described as being of "World War II vintage."

The Carter administration, however, has not given its approval to a Pakistani request for 110 A-7 Corsair light bombers, and this has caused considerable annoyance in military and government circles.