The wife of ousted Pakistani Prime Minister Nawaz Sharif on Sunday captured his parliamentary seat with a reduced majority in a by-election seen as a test of support for the ruling party ahead of the 2018 general election.

Sharif's daughter Maryam said her mother Kulsoom won despite Pakistan Muslim League-Nawaz (PML-N) party workers being threatened and kidnapped. Although she did not name anyone, PML-N sources said she was referring to alleged intimidation by parts of Pakistan's powerful military.

"This is not an ordinary victory," Maryam said in a speech to jubilant PML-N supporters. "You have defeated not only people who were in the field but also those who are invisible."

The main opposition Pakistan Tehreek-e-Insaf (PTI) party made gains but alleged voter irregularities in the eastern city of Lahore, the electoral heartlands of the Sharif family since 1980s.

The PML-N wanted to demonstrate that support for the Sharif family was undiminished despite the Supreme Court's removal of Nawaz, who has kept control of the party and installed long-term ally Shahid Khaqan Abbasi as prime minister.

Maryam said dozens of PML-N activists were blindfolded and picked up from their homes at night, while others received threatening phone calls from unknown numbers during the campaign.

"This victory is a message to the forces hatching conspiracies against Nawaz Sharif that there would be only rules of people and democracy," added Maryam.

Maryam, who some PML-N leaders see as a future leader, spearheaded the PML-N campaign for her mother with fiery speeches denouncing the judiciary. In an interview with Reuters before the vote, she hinted at military involvement in her father's ouster.

Nawaz, who served two stints in power in 1990s until he was deposed in a military coup in 1999, had strained ties with the military during his third stint in power that ended in his ouster, when the Supreme Court disqualified him for failure to declare a monthly salary, equivalent to around $2,700, from a company owned by his son. Sharif denies receiving the salary.

Nawaz Sharif was the 15th prime minister in Pakistan's 70-year history to be ousted before completing a full term. In seven decades, no civilian government has ever completed its term in Pakistan. The country has been ruled by military generals for more than half of its 70-year history and the military unwilling to see its influence challenged.

Tensions between civilian governments and the military have been a constant source of instability in Pakistan, with the military staging coups and running the country for nearly half the time since independence from British colonial rule in 1947.