New Delhi (CNSNews.com) - Thursday's assassination of former Pakistan Prime Minister Benazir Bhutto has shocked politicians and analysts across the region while raising doubts as to whether parliamentary elections, slated for January 8, will now go ahead.

The assassination at a campaign rally in Rawalpindi, which saw at least 20 people killed by a suicide bomber who also shot Bhutto, ignited violent protests across Pakistan.

The worst rioting broke out in Karachi, Pakistan's most populous city, where at least 10 people died as activists angered by Bhutto's death torched vehicles and stores and fired gunshots.

Deaths were also reported in Lahore and elsewhere.

Police used tear gas and batons to break up an angry demonstration in Peshawar after Bhutto supporters blocked a major road in the northwestern city. Billboards promoting the political party associated with President Pervez Musharraf were destroyed by protestors who shouted anti-Musharraf slogans.

In neighboring India, Prime Minister Manmohan Singh said the assassination was "a reminder of the common dangers that our region faces from cowardly acts of terrorism and of the need to eradicate this dangerous threat."

Musharraf, who convened an emergency meeting of top advisors, condemned the killing and appealed for calm in a statement broadcast on state television, announcing three days of national mourning.

"The biggest threat to Pakistan and this nation is from these terrorists," he said. "We will not sit and rest until we get rid of these terrorists, root them out."

But several Bhutto aides said the government had much to answer for, claiming that the authorities had not provided security for Bhutto and that officials had been "dismissive" about her requests in this regard.

Prof. Ramakant, a senior scholar on South Asia, said it would not be unnatural if suspicions were raised as to whether lax security was a deliberate move by the government. He said Islamabad must hold a transparent inquiry into the killing.

Another former prime minister, Nawaz Sharif, also blamed the government for failing to provide sufficient security for Bhutto.

"If Musharraf can spend crores [tens of millions] on his own security, could he not spend some amount on the security of Bhutto?" asked Sharif, who like Bhutto returned recently from exile to contest the election.

Sharif also announced that his party would now boycott the election, adding to uncertainty about whether it will take place. Bhutto's Pakistan People's Party and Sharif's Pakistan Muslim League (Nawaz) are the biggest opposition parties in the country.

Several political groups in Pakistan demanded the formation of an interim national government.

For many in Pakistan and India, Bhutto and Musharraf have come to represent two opposing forces dominant in Pakistan -- Bhutto promoting liberal, democratic ideals and Musharraf a pro-army trend that has long resisted a return to parliamentary democracy.

Musharraf, who recently stood down as head of the army in a bid to defuse a political crisis, is viewed by the U.S. as a key ally in the campaign against Islamist terrorism in the region.

But he has also come in for growing criticism, both inside and outside Pakistan, over an apparent inability -- or unwillingness, some critics say -- to deal decisively with extremists, especially in tribal areas near the border with Afghanistan.

In the days before her death, Bhutto told supporters that terrorism had spread in Pakistan since Musharraf seized power eight years ago in a military coup.

(Cybercast News Service International Editor Patrick Goodenough contributed to this report.)

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