NOWSHERA (Reuters) -- Guarded by hundreds of police, former prime minister Benazir Bhutto took her campaign to Pakistan's restive tribal northwest as the opposition oiled its vote machinery on Wednesday to battle President Pervez Musharraf.

After a vote boycott drive disintegrated, main opposition leaders Bhutto and former prime minister Nawaz Sharif, both returned from exile, organized rallies as a campaign clouded by worries of vote rigging and rising militant attacks geared up.
Underlining the insecurity, pro-Taleban militants killed six Pakistani soldiers in an attack on a military convoy in the northwest near the Afghan border, and 15 insurgents were killed in retaliation, the military said.
More than 800 people have been killed in militant-related fighting since July, military officials say.
“We should not sit as silent spectators while terrorists are killing innocent people,” Bhutto told supporters in the town of Nowshera in North West Frontier Province, where tribal militants are fighting government forces.
Several thousand people chanted “Prime Minister Bhutto” and clapped as she stood to speak from behind a bullet-proof podium.
With the main opposition parties adding some credibility to a January 8 parliamentary election by agreeing to run, political leaders organized their parties before the campaign picks up pace after the publication of candidates' lists on Sunday.
Sharif held rallies in Punjab, Pakistan's most populous province, which returns about half the members of parliament and is his traditional stronghold of support.
“These meetings are kind of warm-up matches,” said Ahsan Iqbal, spokesman for Sharif's party.
The election is essentially a three-way contest between the two main opposition parties and the Pakistan Muslim League (PML), which backs Musharraf.
The election comes amid opposition fears there is too little time before the election for a free and fair vote and that the result will be biased in favor of parties loyal to Musharraf, raising the prospect of a contested result.
The Pakistani media, curbed under emergency rule imposed by Musharraf last month, said authorities were trying to restrict election coverage with a warning not to violate a ban on live broadcasts, or risk three years in jail.
“It's not only a warning but a threat to all TV channels and an attempt to silence the free media,” the Pakistan Federal Union of Journalists said in a statement.