By Faisal Aziz

THATTA, Pakistan, Feb 9 (Reuters) - To chants of "Democracy is the best revenge", tens of thousands of Benazir Bhutto's followers rallied in southern Pakistan on Saturday as her party relaunched an election campaign derailed by her assassination.

Around 2,000 police and hundreds of private armed security guards from Bhutto's Pakistan People's Party (PPP) secured the venue, a sports stadium in the town of Thatta in the southern province of Sindh, amid fears Islamist extremists could seek to disrupt the run-up to a Feb. 18 election.

A suspected suicide bomber attacked an election rally for another secular party, the Awami National Party, in northwest Pakistan on Saturday, killing 14 people and wounding 24.

The PPP rally was the first since Bhutto's Dec. 27 assassination by a suicide bomber in the garrison town of Rawalpindi, adjoining Islamabad, an attack that has stoked fears of instability in nuclear-armed Pakistan and led to a postponement of the vote.

The size of the rally -- police estimated the crowd at over 100,000 -- dwarfed smaller ones Bhutto had held in the run-up to her murder.

"She wanted to change the system and that is why the system has killed her," Bhutto's husband Asif Ali Zardari, de facto party leader, told supporters, some wrapped in red, green and black party flags and wearing hats bearing her portrait.

"The system is her killer, but she knew that even if she lost her life, people like you and me would complete her mission and take revenge," he added.

Giant portraits of Bhutto, Zardari and their 19-year-old son Bilawal, who was appointed party chairman and has vowed to take up his mother's mantle after finishing his studies, hung at the stadium.

Party songs and recordings of Bhutto speeches resonated at the venue. The traditional 40 day mourning period since her killing only ended on Thursday, when thousands gathered at the tomb of the former prime minister.

Bhutto's supporters have accused President Pervez Musharraf of being partly responsible for her death, on the ground that he did not assign her enough security.

The government and the U.S. Central Intelligence Agency (CIA) say Baitullah Mehsud, a Pakistani Taliban militant chief with al Qaeda links based on the Afghan border, was behind the assassination.

CONTROVERSY

British police investigating the murder said on Friday that Bhutto was killed when the blast slammed her head against armour plating around a roof hatch in her vehicle, which she had stood through to wave to supporters.

Aides insist Bhutto was shot before the blast, and want a United Nations probe to find her killer -- something the government refuses. Party officials are now looking to survivors of the Bhutto family dynasty.

"We are PPP workers since birth and will stay that way for ever," said Saleem Rathore, 40, a private contractor.

"Bilawal is Benazir's son and is Bhutto's blood. We can see Benazir's face in him," he added. "He will soon take over the party and will run it the way his martyred mother and grandfather wanted."

Bilawal will not be eligible to run for parliament until he is 25, around the same age his mother was when she stepped into the shoes of her father, Pakistan's first popularly elected prime minister, Zulfikar Ali Bhutto.

The elder Bhutto was toppled and hanged by the military in the late 1970s.

For now, Zardari is running the PPP, though he this week scotched talk that he planned to put himself forward for the premiership. Deputy chairman Makhdoom Amin Fahim, widely seen as Bhutto's No.2, is seen as the party's most likely choice. (Writing by Simon Gardner; Editing by Catherine Evans) (For a Reuters blog on Pakistan, please see http://blogs.reuters.com/pakistan)

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