The damage caused by the truck bomb
Pakistan's president has pledged to fight the "cancer" of terrorism after a suicide bomb killed at least 54 people in the capital, Islamabad.
In a televised speech, Asif Ali Zardari appealed to "all democratic forces" to help to save Pakistan.
The bomb, at the Marriott Hotel, left a six metre (20ft) crater. It is believed to have been detonated in a lorry.
The Czech ambassador was among those killed. The death toll is expected to rise as the wrecked hotel is searched.
Most of the dead were Pakistanis. One Vietnamese, a German and an American are also known to have died.
At least 270 people, including at least a dozen foreigners, were injured in the blast.
Four Britons and an unknown number of Saudi, German, Moroccan, Afghan and US citizens, were among those hurt.
The BBC's Barbara Plett, at the scene of the blast, says emergency services have not been able to reach the upper floors of the hotel, where more people are feared trapped.
The blast destroyed the entire front section of the hotel and brought down the ceiling of the banqueting hall.
Witnesses described a scene of horror as blood-covered bodies were pulled from the wreckage and guests and staff ran for cover from shattered glass.
The police say they suspect it was a suicide attack.
There has been no claim of responsibility so far, but the BBC's Shoaib Hasan, in Islamabad, says the key suspects are the Pakistan Taleban who operate in the north-west of the country.
In a televised address to the nation, President Zardari said he would not be deterred from fighting extremism.
President Asif Ali Zardari vows to rid Pakistan of Islamic militants
"Terrorism is a cancer in Pakistan, we are determined, God willing, we will rid the country of this cancer," he said.
"I promise you that such actions by these cowards will not lower our resolve."
Pakistan's chief adviser on internal security, Rehman Malik, told the BBC the authorities would fight on until "the last terrorist is cleared".
"We think that the safe Pakistan, safe Afghanistan is safety for the world and therefore we will not let them have easy way."
US President George W Bush condemned the attack and pledged assistance.
He said it was "a reminder of the ongoing threat faced by Pakistan, the United States, and all those who stand against violent extremism".
He said the US would "assist Pakistan in confronting this threat and bringing the perpetrators to justice".
The blast is thought to have been caused by more than a tonne of explosives and police are warning that the hotel could collapse.
A huge area of the 315-room hotel remained on fire hours after the explosion.
There are reports that at least 200 people were in the hotel's restaurants when the explosion hit at about 2000 (1500 GMT).
Many of the diners would have been breaking their daytime Ramadan fast.
The Marriott is the most prestigious hotel in the capital, and is popular with foreigners and the Pakistani elite.
The hotel is located near government buildings and diplomatic missions, so security is tight, with guests and vehicles subject to checks.
The Marriott has previously been the target of militants. Last year a suicide bomber killed himself and one other in an attack at the hotel.
Analysts say the attack is the most serious in the Pakistani capital to date and will spark fears about the country's stability in the face of a growing Islamist insurgency.
Pakistan has been a key ally of the US in its "war on terror", but relations have become strained over tactics.
In recent months Pakistan has voiced growing disquiet over US raids targeting militants in its territory, launched from neighbouring Afghanistan.
The attack came just hours after newly-elected President Zardari gave his first speech to MPs, vowing not to allow Pakistan's territory to be violated by terrorists or foreign powers fighting them.
Al-Qaeda and Taleban militants based in Pakistan's north-west tribal region have repeatedly carried out attacks across the border in Afghanistan.
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