A series of bomb attacks on London's transport network has killed more than 30 people and injured about 350 others.
Three explosions on the Underground left 33 dead and an unknown number died in a blast on a double-decker bus.
UK Prime Minister Tony Blair promised the "most intense police and security service action to make sure we bring those responsible to justice".
An Islamist website has posted a statement - purportedly from al-Qaeda - claiming it was behind the attacks.
Mr Blair, who has returned to London from the G8 summit, also paid tribute to the stoicism and resilience of the people of London and said the terrorists must not be allowed to succeed.
Map of where the blasts happened
The Queen said she was "deeply shocked" and sent her sympathy to those affected and the Union flag was flying at half mast over Buckingham Palace.
US President George Bush told reporters at the G8 Summit in Gleneagles that "the war on terror goes on."
All London Underground services have been suspended until at least Friday.
Bus services have resumed in central London (Zone One) with diversions in affected areas. Most mainline train stations are open.
Metropolitan Police Deputy Assistant Commissioner Brian Paddick confirmed 33 people had died in the blasts on the Underground.
He said there were 21 confirmed fatalities following the blast at 0856 BST in a tunnel between King's Cross and Russell Square.
There were seven confirmed deaths after a blast at 0851 BST 100 yards into a tunnel from Liverpool Street station. The train was either a Central Line or Circle Line train.
And at 0917 BST an explosion on a train coming into Edgware Road underground station blew a hole through a wall onto another train in an adjoining platform.
Three trains were thought to be involved and there were five confirmed deaths so far, Mr Paddick said.
He said it was not yet known how many died in the bus blast at 0947 at the junction of Upper Woburn Place and Tavistock Square.
London Ambulance Service said it had treated 45 patients with serious or critical injuries including burns, amputations, chest and blast injuries and fractured limbs.
Some 300 more people were treated for minor injuries including lacerations and smoke inhalation, LAS assistant chief officer Russell Smith added.
In other developments:
The police set up a casualty bureau number on 0870 1566344
The officer in charge of policing the G8 summit said many of the 1,500 Metropolitan Police officers in Scotland would be urgently redeployed to London
New Olympics minister Tessa Jowell said celebrations to mark the homecoming from Singapore of the successful London Olympic bid team have been cancelled
Pope Benedict said the blasts were "barbaric acts against humanity" in a message to the Archbishop of Westminster Cardinal Cormac Murphy O'Connor
London Mayor Ken Livingstone, speaking from Singapore before flying back to the UK, said Londoners would not be divided by a "cowardly attack".
54 state schools were closed in Westminster
Mobile phone services across London were jammed with all major networks reporting problems as people tried to contact relatives and friends. A spokeswoman for Vodafone said the emergency services were being given priority.
Mr Paddick confirmed police were looking into whether the bus blast was the work of a suicide bomber.
But, he added: "It could as easily be an explosive device left on the bus as the work of a suicide bomber. We are not able to determine which it was yet."
He said no warning had been given before the blasts and that no-one had yet claimed to be behind them.
BBC security correspondent Frank Gardner said a previously unknown group calling itself the Secret Organisation Group of al-Qaeda of Jihad Organisation in Europe had claimed to be behind the attacks in a statement posted on an Islamist website.
The group's statement said the attacks were revenge for the "massacres" Britain was committing in Iraq and Afghanistan and that the country was now "burning with fear and panic", he added.
Early reports had suggested a power surge could be to blame for explosions on the Underground but this was later discounted.
Describing the bus blast in Tavistock Square, witness Belinda Seabrook said she saw an explosion rip through the vehicle.
"I was on the bus in front and heard an incredible bang, I turned round and half the double-decker bus was in the air," she said.
She said the bus had been travelling from Euston to Russell Square and was "packed" with people turned away from Tube stops.
"It was a massive explosion and there were papers and half a bus flying through the air." she said.
One caller to BBC Five Live said his friend had seen "the bus ripped open like a can of sardines".