Pakistani investigators are hunting an al-Qaeda cell, based in Islamabad, that is believed to have carried out the bombing of a hotel in the capital that killed at least 60 people.
They believe the attackers constructed the massive truck bomb at a safe house in the capital, since all trucks entering the heavily guarded city are searched at checkpoints.
A senior investigator says it is likely the explosives were smuggled into Islamabad in small consignments from militant strongholds bordering Afghanistan.
Another senior security official says the explosives used in the bombing were like those used in an attack on the Danish embassy in Islamabad in June and on a bus in Rawalpindi in November last year.
Pakistani officials have revealed that 600 kilograms of high-quality explosives were on the truck that tore through the Marriott Hotel in Islamabad.
Among the dead was the Czech ambassador. About 270 others were hurt in the blast, which devastated the Marriott, located at the foot of the Margalla hills in the city centre. The hotel is near government buildings and diplomatic missions and is popular with foreigners and the Pakistani elite.
The attack was staged hours after new president Asif Ali Zardari made his first address to parliament, a few hundred metres from the hotel.
In a televised address to the nation, Mr Zardari said Pakistan was determined to eliminate the "cancer" of terrorism. "We will not be deterred by these cowards, Pakistanis are brave and fearless people, they are not afraid of death."
Security film footage shows an explosives-laden truck ram the security gate. Shots are fired and flames are seen in the cabin moments before the vehicle explodes.
United States and Vietnamese citizens were also killed in the blast, in which at least a dozen foreign nationals were wounded. The Danish foreign ministry said one of its diplomats was missing.
Six Britons and an unknown number of Saudi, German, Moroccan, Afghan and US citizens were among those hurt.
Funerals of those killed have begun to be held in Islamabad. Rescuers are continuing to search for bodies and survivors, but there are fears that more bodies will be found as rescue teams move deeper into the hotel.
There has been no claim of responsibility so far, but the interior ministry said the attackers were linked to Islamist militants in the northwest border region near Afghanistan.
The heavily guarded hotel was attacked at 8pm local time on Saturday, when a truck blew up at the hotel entrance after it was stopped for a security check.
There is no security footage of the main blast because it destroyed the camera, but officials said the vehicle was packed with 600kg of high-quality explosives, as well as grenades and mortars.
Aluminium powder was also used to accelerate the explosion and added to the ferocity of the blaze, officials said.
The force of the explosion created a crater about eight metres deep, and triggered a fire which engulfed the 290-room, five-storey building for hours. Witnesses described a scene of horror as blood-covered victims were pulled from the wreckage and guests and staff ran for cover from shattered glass and flames.
The fire has now burned out and rescue workers have been searching the building room by room, pulling bodies out of the blackened debris. Officials have warned that the building could collapse.
The hotel has been bombed twice before but the Saturday evening blast was the most serious in the Pakistani capital since the country joined the US-led campaign against militancy in late 2001.
Al Qaeda-linked militants based in hideouts in the Afghan border have launched a bloody campaign of bomb attacks in retaliation for offensives by the security forces.
Meanwhile, British Airways has suspended its flights to Pakistan because of security fears after the suicide truck bombing.
The airline, which has six flights a week to Pakistan, says the suspension is temporary and will be reviewed. The last flight from London to Islamabad, the only destination served by the carrier in Pakistan, was on Sunday.
Separately, Pakistani troops fired on two US helicopters that intruded into Pakistani territory on Sunday night, forcing them to turn back to Afghanistan, a senior security official with knowledge of the clash said on Monday.
Worrying time, says Clark
New Zealand Prime Minister Helen Clark says the bombing in Pakistan will be a difficult start for its new president.
Miss Clark says Pakistan has many challenges in front of it, with internal struggle and instability in the border area with Afganistan.
She says New Zealand condemns the bombing, and offers its sympathy to the people and government of Pakistan.
Miss Clark says it is also a very worrying time for members of the Pakistani community in New Zealand, who are fearing for people at home. She says such acts of terrorism are horrific and cowardly, causing death and misery to innocent people.
The Ministry of Foreign Affairs says it is continuing to make further inquiries to establish if any New Zealander has been injured or killed in the bombing. It says there are 29 New Zealanders registered as living in Pakistan, with anywhere between two and seven living in Islamabad.
The former president of the Pakistan Association in New Zealand, Waqar Ali, says what has happened has shocked and saddened the Pakistani community here. He says no one can really believe what has happened.
Mr Ali says he has spoken to his family back in Pakistan, including his wife and children who are holidaying there, and everyone is concerned and worried about the situation.
Cricket tournament postponed
The head of the New Zealand Cricket Players Association, Heath Mills, says its members are relieved not to be in Pakistan at the moment.
The Black Caps would have been playing in Pakistan at the weekend had the Champions Trophy Tournament not been postponed due to security fears.
Heath Mills says the bombing shows the decision to postpone was the right one and New Zealand players he has spoken to are relieved to be at home.
Mr Mills says the thoughts of the players are with the people of Pakistan.