Pakistani security forces arrested two relatives of Zulfiqar Ali Bhutto near the executed former prime minister's grave today and used tear gas and night sticks to break up a defiant attempt by the opposition to breathe new life into the flagging two-week-old civil disobedience campaign against President Mohammed Zia ul-Haq's martial-law rule.

There were scattered demonstrations elsewhere in the volatile Sind Province in southeast Pakistan, but nothing on the scale of that in the first 10 days of the disturbances, which have left 24 dead by official count. The eight-party opposition alliance, the Movement for the Restoration of Democracy, said that more than 60 persons have been killed and several thousand arrested in the largest and most sustained challenge to Zia's six years of rule.

Punjab Province, which comprises two-thirds of Pakistan's 84 millon population, remained quiet for the most part as opposition activists began questioning whether the movement was losing some of its fire in the face of harsh crackdowns by security forces.

Among those arrested today near Bhutto's birthplace and his family's ancestral burial place, about 300 miles north of Karachi, were his cousins Mustaq Ali Bhutto, who had been in hiding for two days, and Sayed Ahmed Bhutto. Police also jailed a student leader of the late Bhutto's now banned Pakistan People's Party, Gada Hussein Quershi, and another party activist, Ghulam Hyder Narajo.

Bhutto was hanged in February 1979, 18 months after being overthrown in a military coup led by Zia.

Elsewhere in the rugged, desert-like interior of Sind Province there appeared to be a relaxation today of the Army's near occupation of many towns and cities. During a day-long tour of the province, correspondents saw Army troops and trucks mounted with heavy machine guns in such continuing trouble spots as Dadu, where mobs repeatedly confronted police and ransacked public buildings. But the troop presence had greatly diminished or even been eliminated in other towns.

The confrontations at Bhutto's grave occurred after hundreds of protesters gathered in this dusty hamlet of mud huts near the elaborately adorned marble tomb and chanted slogans demanding an end to martial law, restoration of the suspended 1973 constitution and the release of political prisoners, including Bhutto's 30-year-old daughter Benazir, who has been under house arrest for several years.

The police had banned today's rally and Army troops last night surrounded the hamlet and sealed off all approaches to it. However, hundreds of villagers from the area and other protesters walked in around the roadblocks through the canal-laced marshy fields of the Indus River floodlands.

One of them, Muhtar Ahmed, a 20-year-old medical student and Pakistan People's Party supporter from Hyderabad, said he walked through muddy irrigation canals and flooded fields for two miles to reach the hamlet.

"You can read everything from these people's faces. They want democracy and they will struggle forever to get it," Ahmed said.

After the rally and prayers at Bhutto's grave, the two cousins and other supporters of the Movement for the Restoration of Democracy walked two miles to a police roadblock and offered themselves for arrest as a gesture of defiance of the martial law authority.

Police prevented reporters from reaching the roadblock, but witnesses said when the crowd did not disperse, security forces fired tear-gas canisters and beat demonstrators with night sticks. The protesters pelted the police with stones and burned Zia in effigy.

Three western correspondents skirted the roadblocks and entered this hamlet through rice paddies at the rear, finding a festive, carnival-like atmosphere with Pakistan People's Party flags and posters of Bhutto everywhere and residents chanting "Death to martial law" and "Long live the PPP."

Later, Mohammed Hassan Bhutto, who is not related to the former prime minister, appeared at the tomb to submit his resignation in protest from the district council of Larkana, 12 miles south of here.

In the center of Larkana, a group of protesting students was dispersed by tear gas. No serious injuries were reported in either demonstration.

Pakistan People's Party workers acknowledged in interviews that the intensity of the Sind protests has diminished, and that they have been disappointed by participation of supporters in the Punjab.

Other Movement for the Restoration of Democracy leaders in Karachi have been expressing concern privately that last week's wave of violence, in which banks, railway stations and other public buildings were burned and looted, reflected a turn in the protest that could discredit the movement.

While publicly blaming the violence on police overreaction to peaceful protest, some of the opposition activists, asking not to be identified, said that in some areas the protest has taken on a Sind nationalist tinge rather than focusing on restoration of democracy.