By Gul Yousufzai

QUETTA, Pakistan (Reuters) - Sixty-four people including school children died on Saturday in a bomb attack carried out by extremists from Pakistan's Sunni Muslim majority, police said.

A spokesman for Lashkar-e-Jhangvi, a Sunni group, claimed responsibility for the bomb in Quetta, which caused casualties in the town's main bazaar, a school and a computer center. Police said most of the victims were Shi'ites.

Burned school bags and books were strewn around.

"The explosion was caused by an improvised explosive device fitted to a motorcycle," said Wazir Khan Nasir, deputy inspector general of police in Quetta.

"This is a continuation of terrorism against Shi'ites."

"I saw many bodies of women and children," said an eyewitness at a hospital. "At least a dozen people were burned to death by the blast."

Most Western intelligence agencies have regarded the Pakistani Taliban and al Qaeda as the gravest threat to nuclear-armed Pakistan, a strategic U.S. ally.

But Pakistani law enforcement officials say Lashkar-e-Jhangvi has become a formidable force.

TENSIONS

Last month the group said it carried out a bombing in Quetta that killed nearly 100 people, one of Pakistan's worst sectarian attacks. Thousands of Shi'ites protested in several cities after that attack.

Pakistani intelligence officials say extremist groups, led by Lashkar-e-Jhangvi, have escalated their bombings and shootings of Shi'ites to trigger violence that would pave the way for a Sunni theocracy in U.S.-allied Pakistan.

More than 400 Shi'ites were killed in Pakistan last year, many by hitmen or bombs, and the perpetrators are almost never caught. Some hardline Shi'ite groups have hit back by killing Sunni clerics.

The growing sectarian violence has hurt the credibility of the government, which has already faced criticism ahead of elections due in May for its inability to tackle corruption and economic stagnation.

The schism between Sunnis and Shi'ites developed after the Prophet Muhammad died in 632 when his followers could not agree on a successor.

Emotions over the issue are highly potent even today, pushing some countries, including Iraq five years ago, to the brink of civil war.

Pakistan is nowhere near that stage but officials worry that Sunni extremist groups have succeeded in dramatically ratcheting up tensions and provoking revenge attacks in their bid to destabilize the country.

(Reporting by Jibran Ahmed; Writing by Michael Georgy; Editing by Stephen Powell)