Altantuya Shaariibuu was shot dead and her body blown up with military-grade plastic explosives in 2006.
Her death was linked to a scandal which allegedly saw kickbacks doled out during a 2002 deal to purchase French submarines, on which Altantuya worked as a translator.
This week, a government bodyguard convicted of her murder made the explosive claim that it was ordered by Najib, defence minister at the time - prompting the veteran politician to head to a Kuala Lumpur mosque to proclaim his innocence.
In front of a cheering, 1,000-strong crowd, the 66-year-old swore an Islamic oath in which he denied that he gave the order and pumped his fist in the air.
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"I stand with the truth, I only fear Allah," said Najib, dressed in a traditional Islamic outfit and alongside his wife.
The former prime minister - whose government lost power last year amid claims of massive graft - has been charged with looting state coffers, but he still commands substantial support among the country's Muslim majority.
Malaysian authorities reopened the investigation into Altantuya's murder last year after Najib lost power.
Two bodyguards were sentenced to hang over the killing. One of them, ex-police officer Azilah Hadri, claimed that Najib ordered him to "shoot to kill" Altantuya in a statement that is part of a legal appeal.
The second, Sirul Azhar Umar, fled to Australia and has said he is willing to assist the new investigation but Canberra is unlikely to extradite him as he is facing the death penalty.
Altantuya's lover Abdul Razak Baginda, a close aide to Najib, is a central figure in the scandal and was accused of arranging the kickbacks. He was also tried over the case but was acquitted.
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