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Former Pakistani Prime Minister and opposition leader Benazir Bhutto was assassinated in a suicide attack after speaking at a campaign rally in Rawalpindi, Pakistan on December 27, 2007. Bhutto's attacker shot her in the neck and chest and then blew himself up next to her car. She was 54. Bhutto is pictured visiting the White House in a June 6, 1989 file photo in Washington. (UPI Photo/Cliff Owen/FILES)

Saying violence has claimed far too many lives in the country, Human Rights Watch's South Asia researcher Ali Dayan Hasan said: "Benazir Bhutto was a democrat who believed in the supremacy of constitutional rule and throughout her career, sought power through the ballot box. Today, she died campaigning for votes and calling for a free election."

HRW urged the Pakistani government to conduct an independent and transparent investigation into the assassination and give it its full cooperation.

"Given Bhutto's public accusations of government threats against her and the absence of judicial independence in Pakistan, an independent and transparent investigation is absolutely essential," Hasan said.