Pakistan's most prominent opposition figure, Benazir Bhutto, is to return to her family home here Wednesday to bury the body of her brother in an emotional political event that will test both the martial-law government and the fractious political opposition.

Whether Bhutto's return from 19 months in exile will signal a new opposition campaign against the rule of President Mohammed Zia ul-Haq, as she has promised, remains uncertain.

Bhutto, 31, daughter of the late prime minister Zulfiqar Ali Bhutto, will accompany the body of her brother, Shahnawaz, to their home town here in Pakistan's Sind Province. Shahnawaz Bhutto's death last month at 27 in France, still not explained, has returned the family, and especially Benazir, to the center of Pakistan's political stage.

The Pakistan People's Party, which Benazir Bhutto heads, has encouraged thousands of people to gather here for the burial in a show of support for the family. This evening, crowds of supporters gathered in and around the family compound, which was festooned with banners proclaiming the martyrdom of the elder Bhutto, who was hanged in 1979, and his son.

Both government and family sources said as many as 100,000 persons might try to attend the ceremonies. Party Chairman N.D. Khan, in Karachi, denied any intent to turn the funeral into a political event. But it is clear that the party wants to use the gathering to demonstrate Benazir Bhutto's popularity.

It remained unclear how far the government was prepared to go in letting its opposition benefit politically from the funeral. The authorities have banned gatherings of more than four persons here but have made no move to break up the crowds already gathered in town.

Interior Minister Shah Mahmoud Khurro said here tonight that "there are no restrictions here. They have a lot of people in the Bhutto family compound, and if she makes a speech, who's going to stop her?"

Khurro also made it clear that authorities would oppose any direct challenge by Benazir Bhutto. He said the government had moved in troops and police "sufficient for any eventuality."

"If she starts agitating and tries to paralyze the government, any government would arrest her," he said.

Benazir Bhutto is a fierce opponent of President Zia, who toppled her father in a 1977 coup and saw him hanged on a murder conspiracy charge two years later. Zia's government held her under house arrest for nearly three years before releasing her in January 1984 for medical treatment abroad.

From her base in London, the Radcliffe and Oxford alumna has continued to campaign against Zia's martial-law rule. Last month, she told an interviewer that she would return home to "take the country by storm or be taken by storm."

Diplomatic analysts and Pakistani politicians predict that Bhutto will avoid a direct challenge to Zia now. Khan said the party leadership would discuss the possibility of her remaining in Pakistan to campaign.

"As long as martial law is in force, it would be a great risk for her to stay," said Khan. Citing restrictions on political activity, he said "anything could be done against her; my own inner feeling is that it may be best if she went back."

Pakistani and diplomatic observers suggest that Bhutto might also find it difficult to make necessary compromises within her party and in the broad opposition coalition that it dominates, the Movement for the Restoration of Democracy.

"In exile, she has not had to align with any of the personal and regional factions which divide the opposition," said a diplomat in Karachi. "If she stays, she will have to choose her allies and her opponents and take specific positions on issues -- all of which could harm her standing," he added.

The opposition is divided on how far to cooperate with the evolution toward civilian rule now being managed by Zia. Although Bhutto and other opposition leaders boycotted the nonpartisan elections held last February, some opposition politicians ran for seats in the National Assembly set up by Zia.

Another leading member of the coalition, Sherbaz Mazari of the National Democratic Party, suggested that Bhutto would have trouble working with opposition leaders from other parties. "She's not very good at compromise," he said.

These diplomatic and Pakistani observers suggested that Bhutto might cite a continuing ear ailment as a reason for returning to her London base and postponing any direct agitation against the government.