By Ilyas Khan
BBC News, Karachi
Suicide bombers have hit the Pakistani capital, Islamabad, on a number of occasions in recent months, but the blast on Saturday was on an altogether different scale.
The man who rammed his truck into the front gate of the Marriott Hotel was sitting on an estimated 1,000kg (one tonne) of explosives, a quantity never before used by militants in attacks inside Pakistan.
The number of dead may rise well above the official toll of 54 once salvage workers get to the rest of the hotel's burned-out rooms.
The Marriott was the first five-star hotel to be built in Islamabad and has been a favourite haunt for diplomats, dignitaries, the city's Westernised elite and well-off foreign visitors.
As such, it has been an obvious target for Islamist militants. Twice in the past it has suffered bomb attacks. On both occasions it sprang right back to business as usual.
But that won't happen this time. The huge truck-bomb set nearly the entire building on fire. It was a blaze that lasted for well over eight hours.
Some reports say the target this time around may have been some US security officials staying at the hotel. But once again it looks like Pakistanis bearing the brunt of militant anger at the United States.
Nearly all of the identified dead so far are Pakistanis.
No militant group has accepted responsibility for the attack. But the obvious suspect is the Tehrik-e Taleban Pakistan (TTP), a loose umbrella organisation of militant groups in the north-western tribal areas, on the border with Afghanistan.
The head of the TTP is the feared militant, Baitullah Mahsud, the man the previous government said planned the assassination of Benazir Bhutto.
He operates from the South Waziristan region which is known for its camps of suicide bombers who are mostly recruited from schools and religious seminaries.
So far nearly all of the identified dead are Pakistanis
Over the years, the TTP has displayed the ability to penetrate high-security zones, often hitting close to military or other high-profile targets.
Saturday's blast occurred in just such a high-security zone. Security all over the capital had been upgraded because the country's new civilian President, Asif Zardari, was giving his maiden speech to parliament.
That event, which took place just four hours before the blast, brought together the entire political leadership of the country, the military top brass and the diplomatic corps under a single roof.
In a brief address to the nation after the blast, President Zardari said: "We are not afraid of death, it will come at the appointed time, but we are determined to clear this cancer [of militancy] from Pakistan."
He called upon "all democratic powers" to help save Pakistan against this threat.
But few Pakistanis expect a quick victory over the militants. In fact, many believe the situation has gone from bad to worse since the spectacular defeat of the allies of former president and military chief Pervez Musharraf in February's general elections.
Red Mosque catalyst
Saturday's attack is the 11th by a suicide bomber in the country this year. It is the sixth in Islamabad since July 2007, when the army carried out a bloody 10-day siege of the city's radical Red Mosque.
More than 100 people, most of them members or backers of a vigilante group raised by a seminary attached to the mosque, were killed in that conflict.
Many analysts consider the Red Mosque confrontation to be the catalyst that sparked the beginning of the end for Pakistan's earlier policy of appeasement towards the Islamic militants in the north-west of the country.
Rescue operations have been hampered as the hotel is too unstable
Before the Red Mosque, the reluctance of the state to fully take on the Islamic militants led to policies that weakened both the tribal administrations and the military, leaving the militants in control of huge areas of the region bordering Afghanistan.
The Red Mosque deaths provided the trigger for certain militant groups - notably those in the eastern parts of South Waziristan and further north in Swat and Bajaur - to decide that the main target was the Pakistani state.
Within four months of the siege, these groups came together in the TTP alliance, with Baitullah Mahsud as its head. Since then, dozens of attacks on military, police and diplomatic targets have shaken the military and embarrassed the government.
During September alone suicide bombers have hit targets in various parts across northern Pakistan, such as Islamabad, Lahore in the east, and Mardan and Charsadda in the west.
These attacks have come in the wake of intensified military action against TTP groups in Swat and Bajaur.
Bajaur has seen the most sustained operation by the military against militants. Hundreds of thousands of civilians have been displaced by the fighting.
The attack is the eleventh by a suicide bomber in the country this year
Added to this volatile cocktail are the recent US missile strikes and one ground operation in the tribal areas.
These have helped the militants regain some of the public sympathy they have progressively lost during the last year. Whatever goes wrong now, many people blame the Americans.
Saturday's attack is interpreted by analysts as indicative of the pressure the militants appear to be under in Swat and Bajaur.
The fact that they are gunning for the Pakistani establishment - and the Marriott Hotel is very much a symbol of that establishment - may also suggest the militants have lost some of their global ambitions they had after 9/11.
The Pakistani government and the militants are also engaged in a battle for the support of ordinary Pakistanis.
It remains to be seen what impact the televised images of the charred wreck of the Marriott, where hundreds of Muslims had been taking their evening Iftar meal during Ramadan, will have across the nation.