The explosion ripped through the heavily guarded Marriott Hotel in Islamabad. (B.K. Bangash/Associated Press)

At least 40 people were killed and 250 others wounded Saturday after a truck bomb exploded in front of the Marriott Hotel in the Pakistani capital, Islamabad, police said.

The bomb was detonated around 8 p.m. local time when restaurants at the five-storey hotel were packed with diners, officials and witnesses said.

The blast was powerful enough to destroy the front of the building and leave a crater 10 metres deep. It reverberated throughout Islamabad and shattered windows hundreds of metres away.

"The fire has eaten the entire building," said Mohammed Ali, an emergency service official at the scene. He said that after an initial chaotic search to find survivors, rescue teams had only been able to make two brief forays inside but found no bodies or survivors and had to retreat quickly.

Teams of firefighters sprayed water from firehoses as bulldozers cleared away debris.

Mohammed Asghar, a worker from a nearby office with a makeshift bandage round his head, said there was more than one man in the truck and that they had argued with the hotel guards.

"Then there was a flash of light, the truck caught fire and then exploded with an enormous bang," he said.

Senior police official Asghar Raza Gardaizi estimated the truck carried more than 1,000 kilograms of explosives. He said it's feared many more people will be found buried in the rubble.

Information Minister Sherry Rehman said at least 250 people were wounded. Hospital staff said 21 foreigners were among the injured, including four Britons, four Germans, two Americans and one each from Denmark, Saudi Arabia, Morocco, Libya, Lebanon and Afghanistan. The Saudi ambassador said several staff from the kingdom's national airline were missing.

Pakistani Senator Tariq Azim Khan said it's possible the hotel was a secondary target. He said investigators speculate the primary targets were the parliament buildings where, about two hours earlier, President Asif Ali Zardari was addressing legislators.

Pakistan's interior minister, Rehman Malik, said security was increased after officials learned of a threat against the parliament two days ago.

Zardari appeared after midnight on state television to condemn the "cowardly attack." He said he understood the victims' pain because he had buried his own wife — assassinated former Prime Minister Benazir Bhutto — in December.

"Make this pain your strength," he said. "This is a menace, a cancer in Pakistan which we will eliminate. We will not be scared of these cowards," he said.

Harper condemns killing of innocent people

In a statement issued by the Prime Minister's Office in Ottawa, Stephen Harper condemned what he called the appalling loss of life and injury to "so many innocent people" and extended Canada's deepest sympathies to their families.

Harper said the use of violence and terror against innocent civilians must never be tolerated.

The statement said no Canadians are known to be among those injured or killed.

U.S. President George W. Bush said the attack was "a reminder of the ongoing threat faced by Pakistan, the United States, and all those who stand against violent extremism."

"We will fully support the democratically elected government of Pakistan and the Pakistani people as they face enormous challenges economically as well as from terrorism," he said.

U.N. Secretary General Ban Ki-moon strongly condemned the "heinous terrorist attack," and said "no cause can justify the indiscriminate targeting of civilians."

With files from the Associated Press