The State Department said yesterday that it has blocked a shipment of tear gas to Pakistan.

More than 170 persons are reported to have died there in the past several weeks in clashes primarily between security forces and opponents of Prime Minister Zulfiqar Ali Bhutto.

"Conditions have changed substantially in the four weeks since the license was issued," the department said yesterday in response to a question about the $68,000 shipment.

The continuing demonstrations against Bhutto's government have raised serious questions on the part of experienced observers, including U.S. analysts, about just how long the embattled prime minister can maintain his hold on power. Shipment of tear gas at such a time could be interpreted as a U.S. endorsement of the Bhutto government.

Pakistani police reportedly are using huge volumes of tear gas in the confrontations with Bhutto's opponents. They claim he rigged the March 7 national elections and are demanding his resignation.

One State Department official stressed yesterday that the tear-gas decision is "an isolated one" and is not an indication of any shift in U.S. policy toward Pakistan.

Delicate negotiations are under way between the two countries on a broad range of issues, including U.S. opposition to Pakistan's purchase of a plutonium reprocessing plant from France and Pakistan's bid to buy 110 American-made A-7 fighter-bombers.

The Carter Administration has requested $98 million in economic assistance for Pakistan for 1978 and some observers believe the figure could go even higher.

The blocked tear gas was ordered by the Punjab state government in Pakistan, reportedly from Smith & Wesson in Springfield, Mass, although the State Department would not give the name of the company involved and officials at the company could not be reached for confirmation.

The orginal order, for tear gas cannisters and launchers, was placed before the March 7 elections, but a State Department decision on the required export license is understood to have been delayed until after the election so as not to give the impression of trying to influence the voting.

The license was issued on March 17, after riots and demonstrations against Bhutto had begun but before there was an indication that his government could be in jeopardy.

The issue remained dormant until the Punjab government threatened last week to invoke penalty clauses against the U.S. company unless the shipment were made within three days.

The State Department made the decision to suspend the export license and half the shipment late Monday.

(Thousands of Bhutto's supporters demonstrated in Lahore yesterday. Reuter reported. A large crowd of Bhutto opponents gathered in the city center later in the day. In Lyallpur, four opposition supporters were injured by tear gas shells in a clash with police.)