President Mohammed Zia ul-Aq tonight blamed an anti-Pakistan conspiracy by unpatriotic politicians and a foreign government for the 13-day hijacking of a Pakistan International Airlines plane with more than 100 persons on board.

In a radio-TV report to the nation less than 24 hours after the hostages were freed in Damascus, Syria, Zia said the conspiracy was designed to punish Pakistan for standing in the way of another nation's "foreign designs" -- persumably, according to diplomatic and Pakistani source here, the Soviet Union's 15-month occupation of neighboring Afghanistan.

In what most observers here saw as an obvious attack on members of the Pakistan People's Party of executed former prime minister Zulfiqar Ali Bhutto, Zia said the "snakes in the sleeve" inside Pakistan belong to a group that is opposed to Pakistan's ideology and believes in "the politics of violence." Neither the Soviets nor the People's Party were mentioned directly in the Zia speech.

Most Western diplomats questioned here said they do not believe the Soviets played a direct role in the hijacking. There is evidence that the Afghans aided the sky pirates when the plane landed at Kabul and that both the Soviet Union and Afghanistan attempted to use the hijacking to force Pakistan and other Islamic nations to recognize the government in Kabul.

Zia signaled a further crackdown on civilian politicians -- especially members of Bhutto's party -- when he promised "to hold accountable" those who believe in "the politics of violence."

His military government has already arrested hundreds of politicians since the March 2 hijacking. The government has released no information on the arrests, but one usually reliable source placed the figure at about 500.

Most of those jailed were People's Party leaders, including Bhutto's widow Nusrat and daughter Benazir, who now lead the party. Others reported under arrest included leftists of other parties, journalists and some intellectuals.

The arrests followed a crackdown last month on politicians after the formation of an alliance called the Movement for the Restoriation of Demanded elections and the end of Zia's 3 1/2 years of military rule.

According to observers here, it appears that both the embryonic political movement and Bhutto's party lost support as a result of the hijacking and Zia emerged at least to less popular than he was before the plane was seized.

Zia apparently has succeeded in tarring the People's Party terrorist brush -- made easier since the hijackers called themselves "Al Zulfiquar" after Bhutto and since his son Murtaza was seen by Western diplomats moving freely about the plane when it landed in Kabul.

Illustrting an apparent loss in the popularity of the Bhutto family, there was no out-cry or demonstrations accompanying the arrests of the widow and daughter of the former prime minister, who was executed by the Zia government almost two years ago after being convicted of political murders.

Many observers here, however, disagree with the Pakistani government analysis that Al Zulfiquar is the terrorist wing of the party. These observers, Pakistani and Western, say Bhutto's son Murtaza holds more extremist political views than his mother or sister.

Zia appears to be trying to make the most politically out of the release of the hostages. He announced tonight that the Moslems aboard the plane will be offered a trip to Mecca on the way home.

The president denounced the 54 prisoners who he traded for the hostages as not being worth keeping in the country. Although the government has released no names, many of them are believed to have been jailed People's Party members.

Zia also attacked Afghanistan, saying its officials cooperated with the hijackers by giving them weapons and food and encouraging the killing of one of the passengers.

Diplomats said Afghans were attempting to force Pakistan to recognize the legitimacy of their Soviet-installed government.