Thousands of Pakistanis gathered here today to bury the son of former prime minister Zulfiqar Ali Bhutto and demonstrate their opposition to the martial-law rule of President Mohammed Zia ul-Haq.

Benazir Bhutto, Pakistan's most prominent opposition leader, accompanied the body of her brother, Shahnawaz. Returning from 19 months in exile, she said she had not decided whether to stay to lead a renewed campaign against the government.

Regulations formally ban gatherings of more than four persons, and extra police and troops were moved into the area. Those forces were out of sight, however, and no attempt was made to restrict the crowd. Bhutto supporters raised posters and banners calling for political opposition to the government, but the ceremonies were kept free of overt political content.

Benazir Bhutto, during both a press conference and a brief speech to supporters, avoided direct attacks on the Zia government.

Supporters of Bhutto have referred to the uncertain cause of her 27-year-old brother's death last month on the French Riviera, implying that it might have been a politically motivated murder. But she said she would await results of a French autopsy, which may clear up speculation that Shahnawaz died of a drug overdose or was poisoned.

Although Bhutto and her supporters constantly referred to her brother as a "martyr," she stopped short of accusing Zia's government of his death.

In statements over recent months from her voluntary exile in London, Bhutto, 32, had said economic problems in Pakistan as well as the continuation of martial law and its associated restrictions on political activity would require her to return soon to revitalize the Pakistani opposition movement. But she told reporters tonight that her brother's death had brought her back unexpectedly early.

"I have not yet decided whether to stay" beyond a traditional 40-day mourning period for her brother, she said, following funeral prayers at the Bhutto home. Seated amid a room full of women who had come to offer traditional condolences, she insisted, however, that "I am more committed than ever before."

Motioning to an elderly woman who leaned, weeping, against her, Bhutto said, "The government hanged her son this past June . . . . There have been too many sacrifices." Bhutto later said the funeral had left her "drained." She said a series of "imponderables," including her uncertain legal status, had prevented her from making definite plans.

In 1984, the government released Bhutto from house arrest for medical treatment abroad, and the federal interior minister said yesterday that all charges against her since had been dropped.

Separate crowds of men and women who pressed around the Bhutto family home here included old-line activists of the Pakistan People's Party founded by Ali Bhutto and now led by his daughter. They mixed with militant students calling for armed revolution and residents of the surrounding region who came to express their loyalty to the Bhuttos, an aristocratic land-owning family in this part of Pakistan's Sind Province.

As many as 10,000 persons may have gathered at one point in the nearly 100-degree heat to intone funeral prayers at a soccer stadium. The overall crowd was much larger than that, but it was difficult to estimate and well short of party predictions of 100,000.

Long motorcades raised thick clouds of dust as they escorted Shahnawaz Bhutto's body from the airport to the family's villa and then to its mausoleum in a nearby hamlet. Ali Bhutto's grave, set among the mud-brick houses of the rice-farming village, has become something of a shrine, where peasants make prayers and offerings.

The emotions raised by the funeral illustrated the populist aura that continues to surround the Bhutto name in Pakistan, eight years after Ali Bhutto fell to Zia's coup d'etat. Bhutto, executed in 1979 after conviction of conspiracy to murder, is still seen here as a people's politician, an image that has devolved on his daughter.

"Given the economic problems over which his government presided, it is not obvious that Bhutto really improved the lot of the common man," a western political analyst said earlier this week. "But Zia is seen as ruling in the interest of a narrow elite -- mostly military -- and most Pakistanis are looking desperately for someone who will govern in the interests of the lower classes," the analyst said.

The question of when and how to revitalize their campaign against Zia is seen as both difficult and critical for Benazir Bhutto and the entire opposition movement.