Pakistani police have made four arrests following a suicide bombing that killed at least 50 at a mosque near Peshawar, in the north-west, local media say.
Two of those arrested were Islamic students and the other two their guests from Afghanistan, a local journalist told the BBC.
The arrests took place at an Islamic seminary, four kilometres (2.5 miles) from Charsadda, scene of the blast.
Former Interior Minister Aftab Sherpao was in the congregation but was unhurt.
Forensic scientists have been combing the remains of the explosive device and the victims, who were part of a large crowd offering prayers for Eid al-Adha.
It was the second suicide bombing in eight months apparently targeting Mr Sherpao - a close ally of President Pervez Musharraf.
The violence comes as Pakistan prepares for a general election next month.
'Abhorrent act'
Worshippers at the mosque in North West Frontier Province were celebrating Eid al-Adha, the Festival of Sacrifice, when the suicide bomber blew himself up amongst the 1,000-strong crowd.
" There was blood and body parts everywhere¿ People were running. Some people were injured in the chaos "
Police said the bomb contained 6-8kg (13-17lb) of explosives and was packed with nails and ball bearings to maximise casualties.
"There was blood and body parts everywhere... People were running. Some people were injured in the chaos," said Iqbal Hussain, a police officer in charge of security at the mosque.
Mr Hussein said he did not know how the bomber managed to gain entry to the mosque because the congregation had been forced to pass through metal and explosive detectors.
After the victims and dozens of wounded were removed from the mosque complex, police and forensic scientists began carefully sifting through the rubble, as well as the bloody shoes and clothes left behind.
President Musharraf later issued a statement ordering the security forces to "track down the masterminds behind this abhorrent act".
"No Muslim could even think of committing such a heinous crime," he was quoted as saying by the Associated Press of Pakistan news agency.
"A handful of extremists were bent upon forcing their way of distorted thinking on the vast majority of moderate and practicing Muslims, which was totally unacceptable," he added.
Islamist militants suspected
The BBC's Barbara Plett in Islamabad says the likeliest suspects are pro-Taleban militants, who have stepped up attacks against the government in retaliation for operations against them in the border areas near Afghanistan.
More than 600 people have been killed during the past six months, including about 200 soldiers, in violence sparked by the ousting of armed militants from the radical Red Mosque in Islamabad in July.
As interior minister in the government recently dissolved by Mr Musharraf ahead of the 8 January election, Mr Sherpao was the country's top security official and led the campaign against militants.
The threat of growing Islamist violence was cited by Mr Musharraf when he declared a state of emergency in Pakistan last month. He ended emergency rule last Saturday, however, saying the threat had been contained.
US Defence Secretary Robert Gates said in the wake of the latest bombing that al-Qaeda had "re-established itself" in the border areas and begun to focus on attacking the Pakistani government.
"Al-Qaeda seems to have turned its face toward Pakistan and attacks on the Pakistani government and Pakistani people," he said.
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