Pakistan's Taliban claimed responsibility for the blast and said it would launch attacks in the United States and Europe "very soon" - repeating a threat to strike Western targets in response to drone attacks that have targeted its leadership.
In Washington, the White House condemned the Quetta attack on a Shi'ite rally and expressed solidarity with the Pakistani people, saying it was "even more reprehensible" because it came during the Muslim holy month of Ramadan as Pakistan reels from disastrous flooding.
A US counterterrorism official said the threat by the al-Qa'ida-linked Taliban against the United States and Europe could not be discounted.
The attack came just two days after Washington added the Pakistani Taliban to its list of "foreign terrorist organizations" and charged its leader, Hakimullah Mehsud, with plotting a bombing that killed seven CIA agents at a US base in Afghanistan last December.
In Quetta, dozens of dead and wounded people lay in pools of blood as fires engulfed vehicles. Senior police official Hamid Shakeel told Reuters at least 54 people were killed and about 160 wounded.
Hours later, the Taliban said the bombing was revenge for the killing of radical Sunni clerics by Shi'ites, further challenging Pakistan's unpopular civilian government.
"We take pride in taking responsibility for the Quetta attack," Qari Hussain Mehsud, a senior Pakistani Taliban and mentor of suicide bombers, told Reuters.
Earlier in the day, the Taliban also claimed responsibility for bombings on Wednesday at a Shi'ite procession in the eastern city of Lahore in which at least 33 people died.
Those blasts were the first major attack since the worst floods in Pakistan's history began more than a month ago. The Taliban and its allies often target religious minorities in a campaign to destabilize the government.
Aside from its battles against homegrown Taliban, Pakistan is under intense American pressure to tackle Afghan Taliban fighters who cross the border into Pakistan's lawless tribal areas to attack US-led NATO troops.
The United States has stepped up missile strikes by pilotless drone aircraft against militant targets in Pakistan's Pashtun tribal lands since the start of 2010.
US drones yesterday fired missiles at two targets in the North Waziristan tribal region, killing seven militants, including two foreigners, intelligence officials said.
Pakistan's Taliban has responded to drone attacks by saying it would strike Western targets.
"We will launch attacks in America and Europe very soon," Mehsud told Reuters by telephone yesterday from an undisclosed location.
The group claimed responsibility for a failed bomb plot in New York's Times Square in May and, in December 2009, a Spanish court jailed 10 Pakistanis and an Indian for attempted suicide bombings on Barcelona's metro in 2008, saying they were inspired by the Pakistan Taliban's then leader.
"No one is discounting the threat they pose and we and our partners are working hard to disrupt their terrorist activities," a US counterterrorism official said on condition of anonymity.
US State Department spokesman P.J. Crowley said the group was designated a "foreign terrorist organization" because "it is a threat to the United States but most importantly a threat to Pakistan itself."
Pakistan has said the army would decide when to carry out a full-fledged assault in North Waziristan, where Washington says the militants enjoy safe havens.
US Defense Secretary Robert Gates, visiting troops in Afghanistan, said the flooding "is probably going to delay any operations by the Pakistani army in North Waziristan for some period of time."
In another attack in the northwest, a suicide bomber killed one person outside a mosque of the Ahmadi sect, who consider themselves Muslims but whom Pakistan declares non-Muslims.
Islamist charities, some linked to militant groups, have joined in the relief effort for the millions of people affected by the floods. US officials are concerned the involvement of hardline groups in relief work will undermine the fight against militancy.
Anger is spreading over the Pakistani government's sluggish response to the disaster, raising the possibility of unrest.
Pakistan is also facing economic catastrophe, with the floods causing damage the government has estimated at $43 billion (£28bn), almost a quarter of the south Asian nation's 2009 gross domestic product.
The International Monetary Fund will give Pakistan $450 million (£290m) in emergency flood aid and disburse funds in September to help the economy cope with the devastation.
Talks in Washington with a delegation led by Pakistan's Finance Minister Abdul Hafeez Shaikh on the terms of an $11 billion (£7bn) IMF loan program left him satisfied with the country's commitment to reforms, IMF chief Dominique Strauss-Kahn said.
Under the 2008 IMF loan program, Islamabad promised to implement tax and energy sector reforms and give full autonomy to the State Bank of Pakistan.
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