Leaders of Pakistan's opposition parties said today they are confident of a successful voter boycott of the planned national referendum to extend President Mohammed Zia ul-Haq's rule for five years, but said that they expect Zia to manipulate the results and stay in power anyway.

The opposition leaders described as a trick Zia's call for a vote on the question of whether Pakistanis support the process of Islamization that the military ruler began when he seized power seven years ago. They said voters will not be confused by the ambiguous phrasing. In a televised announcement Saturday night that took the opposition by surprise, Zia said that if a majority of voters endorse the Islamization process in the Dec. 19 balloting, he will remain in the presidency for a five-year term.

"The cat is out of the bag. The holy name of Islam has been used for seven years, and everyone has seen it has done nothing to help develop the country. We do not need a referendum to tell the outside world that Pakistan is an Islamic country," said Ghulam Mustafa Jatoi, leader of the 11-party opposition alliance, the Movement for the Restoration of Democracy.

Jatoi, former chief minister of Sind Province under the Pakistan People's Party government of executed prime minister Zulfiqar Ali Bhutto, said the movement would issue a formal declaration Tuesday calling on Pakistan's 34 million voters to stay away from the polling booths on referendum day.

"The name of Islam is being used. Everybody is a Moslem here. Nobody needs a license. A referendum is held on a specific issue, not to elect a president," said Jatoi, who last year led massive and frequently violent protests in Sind after Zia announced he was postponing promised parliamentary elections until March 1985.

When he deposed Bhutto in a military takeover in July 1977, Zia promised to hold democratic elections in 90 days.

Zia will remain as president and chief martial law administrator even if voters deliver a negative verdict on Dec. 19, Pakistan's information minister, Raja Zafarul Haq said Monday. Speaking to foreign correspondents in Islamabad, Haq said rejection by the voters would mean that "the situation as it obtains today continues . . . until such time as he holds elections and transfers power," Reuter reported.

After the elections, Haq said, "martial law will gradually be lifted." He refused to give a date for the completion of the process, Agence France-Presse reported.

Jatoi, who was imprisoned for more than a year and released on Oct. 27 from three months' house arrest, noted government plans to impound the referendum ballot boxes for three days before announcing the results -- "just to manipulate and rig the results," he said.

Proof of rigging, Jatoi said, "will lie in the fact that no people will be at the polls. They will be empty. They will have to stuff the boxes with their own results."

Other opposition leaders charged that Zia hopes to dupe unsophisticated voters in this fervently religious nation of 90 million people into believing they have a choice of endorsing or rejecting Islamic principles. They noted a government announcement that the ballots will be white, but that the space for a "yes" answer to the referendum question will be printed in the Islamic color, green.

Shah Farid ul-Haq, vice president of the orthodox Moslem Jamiat Ulema Pakistan Party, which is part of the opposition, said, "It is a fraud being played to fool Pakistanis. He Zia has no sanction of the people and no right to an election. Islam does not allow such people to rule."

Jamiat Ulema Pakistan, which claims to represent 60 percent of the country's majority Sunni sect of Islam, has consistently criticized Zia for introducing cosmetic Islamization reforms and for not expanding the jurisdiction of sharia courts that would strictly adhere to codes of Islamic law set forth in the Koran.

"If a person puts down a no, it will be a no to Zia ul-Haq, not to Islam. No one is Pakistan is against Islam. But even if the referendum is rejected, he will declare 92 percent votes for it. No one will know the real result," Farid said.

In random interviews, Pakistanis of varying walks of life said they did not not regard the referendum as a poll on the merits of Islamization, but as a vote to extend Zia's rule.

"He came for 90 days. He wants to stay for 90 years," said an executive of a textile manufacturing firm. Like many critics of the president, he requested that he not be identified.

Several merchants interviewed said they felt the referendum has nothing to do with Islamization, but noted that they intend to vote affirmatively anyway because their businesses have prospered under relatively stable conditions during Zia's martial-law regime.

Western diplomatic sources, while describing Zia's call for a referendum as "deft," noted that U.S. officials, including members of congressional fact-finding teams that have visited here recently, have been prodding Zia to revive democratic institutions, and that a popular referendum partly answers those requests.