The return of Pakistani opposition leader Benazir Bhutto is at once the boldest indicator of political liberalization in this country and the strongest challenge to the fragile process.

The past few days' tumult here in Punjab Province -- as hundreds of thousands of Pakistanis have rallied towelcome Bhutto back from exile and chant for the ouster of President Mohammed Zia ul-Haq -- would have been unthinkable only a few months ago. Last fall, Zia's military administrators were holding Bhutto under house arrest for having planned to address a small meeting of her supporters.

Zia overthrew Bhutto's father, former prime minister Zulfiqar Ali Bhutto, in 1977 and ruled Pakistan under martial law for more than eight years.

Apparently understanding that popular resentment was growing against his military rule and the Army, however, Zia spent last year carefully building a conservative civilian administration through which he could continue to run the country while allowing a controlled degree of political freedom.

Like most of Pakistan's other opposition leaders, who remain outside the new civilian government, Benazir Bhutto objects to Zia's requirement that liberalization include his staying in control until at least 1990. But -- as she cranks up her campaign to force Zia to resign and allow immediate elections -- only Bhutto is regarded as a serious threat who could ultimately provoke Zia or other Army leaders into snatching back absolute power.

Precisely how acute the risk is to Zia's cautious liberalization remains unclear. Even participants in the drama are uncertain how much popular dissent Bhutto will produce or how much Zia will tolerate.

Because Pakistan is the only strategic U.S. ally between Turkey and Thailand, and given the Soviets' occupation of neighboring Afghanistan, the Reagan administration is concerned about Zia's stability. The United States quietly encouraged and then publicly applauded Zia's shift to civilian rule as a way to democratize and stabilize his regime.

In March 1985, Zia called nonpartisan elections for a National Assembly from which he would choose a government to take over the administration. Bhutto's Pakistan People's Party and several other parties called for a boycott of the elections, but more than 50 percent of the voters turned out anyway, electing a largely conservative assembly.

When Zia lifted martial law last Dec. 30, he kept the authority to reimpose it whenever he chooses. But the signs are that Zia could not easily rebottle the impatient political energies that he uncorked in ending military rule -- and that if he tries, he will further polarize Pakistani politics and ultimately destabilize his own rule.

"I don't think the trend can be reversed," a western political analyst said.

Zia's presidential powers and his continuation as Army chief of staff left many Pakistanis skeptical about how much would change in the country. But three and a half months later, the changes have clearly begun.

Military courts -- which raised concerns among human rights organizations over harsh punishments and the absence of appeals -- have been closed, although political prisoners remain. The country's independent press now freely reports criticisms against the government, although the authorities retain the power instantly to revoke any newspaper's publishing permit.

Mohammed Khan Junejo, picked by Zia to be prime minister, now appears in control of day-to-day government policy and even has removed a number of ministers from the Zia-appointed Cabinet he inherited in December.

"Junejo is much stronger than anyone would have believed a year ago," a western political analyst said recently, "and he was the one giving specific guarantees that Benazir would be able to move freely if she came back."

Bhutto's triumphant return, 101 days after the end of martial law, was only one sign of the enthusiasm with which Pakistanis have resumed party politics. Residents of Karachi, the Indian Ocean seaport that is Pakistan's largest city, say a municipal park has become a weekly political rally site, and the same flurry of politicking has enlivened other towns and cities.

Arif Nizami, editor of the Urdu-language national daily newspaper Nawa-i-Waqt, said last weekend, "I have not seen this level of democracy before in Pakistan.

"You can go to a public meeting and listen to the speaker, and then walk home without the fear of being caught by police with lathis (batons) or tear gas or getting beaten by thugs," Nizami said.

If she is to pose a serious political threat to Zia, Bhutto first will have to assert control over her own party. She was named its acting chairman by her mother, who was given the title by Ali Bhutto shortly before his execution.

The People's Party embraces Pakistanis of varying political views, including confirmed radicals and wealthy, traditional landholding families like the Bhuttos. Benazir Bhutto, who has led the party only from exile, has been trying to gain control of its apparatus by naming her own generally more leftist loyalists in place of older and more established party leaders.

At a press conference last Friday, Bhutto offered no apologies for the irony that, amid her campaign for increased democracy in Pakistan, she leads a party that has never had internal elections. She said she plans to hold them soon.

In the meantime, she is using her absolute authority to appoint party officials who, according to a local journalist, "seem qualified only by being loyal without question to Benazir."

Pakistani newspapers have reported that the party chief in Bhutto's native Sind Province, Ghulam Mustafa Jatoi, was considering breaking away to form a new party because of Bhutto's appointments and what Jatoi himself called her "dictatorial tendencies."

Bhutto also will have to broaden her appeal beyond her organized party, a task that will be both helped and hindered by her political inheritance. What political opinion polls exist in Pakistan suggest that there are about equally large groups of voters either strongly supportive of or sharply opposed to her party.

Even though her father failed to radically improve the lives of Pakistan's poor, his populist commitment and style have left the Bhutto name a revered icon for many Pakistanis. But others regard the family with mistrust and even hatred. In the southwestern province of Baluchistan, her father is most remembered for the brutal suppression of a local uprising while he was prime minister.

The Pakistani economy, riding on bumper cotton and wheat harvests, and continued heavy remittances from expatriate workers in the Persian Gulf states, will work against Bhutto and in favor of Junejo, according to several observers. Having taken over a government installed from above by Zia, Junejo is using the vehicle of the Muslim League -- the party fathered by Pakistan's founder, Mohammed Ali Jinnah -- to build his own base of support among traditional conservative leaders and the public.

Junejo and Bhutto fought an old-fashioned battle of promises for the poor last week, with Junejo visiting Lahore to offer land redistribution to tenant farmers and urban squatters, while Bhutto hastened to promise a monthly minimum wage of about $60, plus land and jobs.

For the moment, Junejo's stance and the discretion with which police have been deployed at Bhutto's rallies demonstrate that the government wants to avoid a confrontation with Bhutto, hoping that her campaign will stall amid the divisions in the People's Party and the rest of the opposition.

Although Bhutto's rhetoric calls for Zia's resignation "immediately," she speaks of the possibility of a drawn-out campaign against him. Conversations with workers and officials from her party suggest that they are willing to let her set the pace for their challenge to the government, although her rallies have included angry young militants who are clearly of no mind to wait.

In the nationwide series of rallies she is using to launch her campaign, Bhutto could easily build a public expectation for quick action on her part -- an expectation she would then be under pressure to fulfill.

If she moves too far, Bhutto will confront Zia with a difficult question of how to respond. If, provoked by her political agitation, Zia has Bhutto arrested or exiled, he would undermine -- and possibly lose -- Junejo, his chosen architect for the civilian rule experiment.

If Zia were to act too slowly, some observers fear that others in the Army might step in. Analysts disagree on the extent to which Zia may face resistance from conservative officers who oppose the civilian rule in the first place.

One experienced western analyst argues that Zia's style of seeking consensus among his colleagues works against any surprise uprising by dissenting right-wing officers. "Zia is a committee man, and has stayed in the mainstream of the Army's thinking," the analyst said. "He helps shape the consensus, but doesn't move beyond it."

The analyst, who asked not to be further identified, argued that "the officers agreed that their administrative mission in running the government was detracting from their military role -- and that is something that bothered them at a time of tension on two borders" with Afghanistan and India.

"It would take the threat of a civil war before a lot of officers would agree with the need to go back," he said.

Other observers, expecially in the Pakistani opposition, are less sure. One Pakistani analyst, Dr. Hasan Rizvi, writing in the Pakistani Defense Journal, noted that, unlike the two previous periods of martial law, Zia's military rule included an expansion of the military's role in national life.

Zia suggested, said Rizvi, "that the military must safeguard the nation's ideological frontiers in addition to its traditional role" of defending the nation's territory. That observation suggests an increased readiness over past years to respond to a political challenge such as that posed by Bhutto.