(Adds governor's comments, updates toll)

By Yousuf Azimy

BAGHLAN, Afghanistan, Nov 7 (Reuters) - Afghans began three days of mourning on Wednesday for 52 people, many of them children, killed in the country's worst suicide attack.

The blast, in the relatively peaceful north, has shaken public confidence in the ability of the government and the 50,000 foreign troops in the country to provide security more than six years after the Taliban were ousted from power.

"In the very miserable incident which took place yesterday, six of Afghanistan's hard-working, honest members of parliament were martyred, and Afghan people including school teachers, students and children were also martyred, and many were wounded," President Hamid Karzai told a news conference in Kabul.

The governor of Baghlan where the attack took place said the death toll had risen to 52 and about 100 people were wounded.

The Taliban have carried out more than 130 suicide bombings in Afghanistan so far this year, but the insurgents denied responsibility for Tuesday's attack on visiting parliamentarians as they were being greeted by schoolchildren and elders.

The bomber approached the parliamentary delegation on foot as children lined up to welcome them on a visit to a sugar factory in Baghlan. Large crowds had also turned out to see the deputies.

There were still pools of blood at the site on Wednesday morning as police collected body parts and put them in plastic bags. School notebooks and children's sandals lay strewn on the ground.

"We are treating the wounded and the condition of some is very critical," said Dr. Mohammad Rokai at the local hospital. "The dead and wounded are mostly children."

BULLET WOUNDS

Baghlan residents peered glumly at the bomb site from behind police cordons. "One of my brothers is missing, he's 12 years old. We don't know if he's alive or dead," said Shafiqullah.

Some of the dead and wounded appeared to have suffered bullet wounds and some residents said Afghan security forces began shooting wildly after the blast.

"This attack was carried out by the Taliban, but only 10 people were killed by the blast. The rest of the victims are from gunfire from the security forces," said Abdul Qadir, pointing to what appeared to be a bullet hole in his dead son's neck.

Other Baghlan residents made similar charges.

The Baghlan governor said authorities were investigating accusations that police had opened fire after the attack.

A Taliban spokesman said the insurgents were not behind Tuesday's attack. The rebels usually distance themselves from attacks that largely kill civilians.

The insurgents' strategy is aimed at convincing Afghans that Karzai's government and its Western backers are unable to bring security to the country, which has already suffered nearly three decades of almost constant war.

As often happens after suicide attacks, many ordinary Afghans blamed the government for failing to prevent the bloodshed.

"I lost my cousin in this attack," said shopkeeper Sakhi Ahmad. "He was a school student and had gone to the ceremony. We will never forget this tragedy and we ask the government to find the culprits instead of announcing days of mourning."

The United States condemned the bombing.

"The terrorist attack today in Afghanistan is a despicable act of cowardice and it reminds us who the enemy is -- extremists with evil in their hearts who target innocent Muslim men, women and children," said White House spokeswoman Dana Perino.

Northern Afghanistan has been relatively peaceful and prosperous compared with the south and east, where Taliban suicide attacks are all too common and insurgents are locked in almost daily battles with Afghan and foreign forces.

NATO commanders say the Taliban are not a unified organisation, but a number of factions operating under loose guidelines handed down from a governing council. Al Qaeda and at least one other insurgent group are also active in Afghanistan. (Additional reporting by Tahir Qadiry in Mazar-i-Sharif and Hamid Shalizi in Kabul, writing by Jon Hemming, editing by David Fogarty)

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