Key Points:

Dramatic CCTV footage released last night by Pakistani authorities shows the moments before a truck containing more than 1,000lb of explosives tried to ram the gates of an Islamabad hotel before blowing up, killing at least 53 people.

The grainy images show a small explosion inside the cab of a dumper truck after security guards refused to allow the driver entry to the grounds of the Marriott hotel.

Panicking guards can be seen trying to put out the resultant fire before sniffer dogs alert them that something is wrong.

The guards can then be seen fleeing the area but the subsequent blast is not captured as it destroyed the cameras.

"The truck was stopped at the barrier and there was an altercation between the attacker and the guards," said Rehman Malik, Pakistan's Interior Minister.

As Pakistan sought to recover from one of its most shocking terror attacks to date and as the death toll from Saturday evening's blast continued to rise, Mr Malik said investigators believed the bomb had contained an estimated 600kg of high-grade, military-style explosives.

"This is the biggest explosion in Pakistan's history," he added.

"The diameter of the crater [left by the bomb] is 69ft."

The Czech ambassador, Ivo Zdarek, was among the 53 people known to have been killed.

More than 260 others were injured and rescue crews were still combing through the smouldering debris last night.

Among the injured were six Britons, including a child and three diplomats.

The Foreign Office said two Britons remained in hospital.

While three other foreigners were among the dead, many if not most of those killed are believed to have been members of the hotel's staff.

It was revealed yesterday that the authorities learnt that militants may have been planning an attack in Islamabad three days earlier.

However, efforts to protect against such an event appear to have been concentrated on the parliament building from where Pakistan's newly elected President, Asif Ali Zardari, delivered a speech on Saturday evening, barely a mile from the hotel.

There was disagreement among officials whether the bomber had targeted the hotel after failing to get close enough to the parliament building.

While there has been no claim of responsibility so far for Saturday's attack, the authorities believe it was almost certainly the work of Islamic extremists based in the country's tribal areas.

Militants from the Pakistani Taleban and al Qaeda have been behind an upsurge in violence in recent months of which Saturday's bombing was just the latest incident.

Pakistani civilians have made up the overwhelming majority of the victims.

Mr Malik said a team of investigators has been examining the hotel and the deep crater in the road outside.

The action of the guards will likely be a key part of the inquiry, given that the owner of the Marriott accused them of a serious lapse by allowing the truck to approach unchallenged.

"If I were there and had seen the suicide bomber, I would have killed him. Unfortunately, they didn't," said Sadruddin Hashwani.

The blast - which used aluminium powder that created temperatures of more than 400C - is just the latest challenge for Pakistan's new civilian leadership.

The government and Mr Zardari have been involved in a war of words with the United States over military operations carried out by American troops inside Pakistan as they step up the hunt for Osama bin Laden before George Bush leaves office.

Mr Zardari condemned the bombing as a "cancer" before flying to New York for a meeting with Mr Bush and talks at the UN where the issue of Pakistan's efforts to confront extremists along its border with Afghanistan is certain to be discussed.