Millions are expected to respond to the call by the Mayor of London, Ken Livingstone, to make a "defiant" gesture against terrorism in the two-minute silence. Residents of the 25 European Union states are expected to show their solidarity by doing likewise.
In London, buses and taxis will interrupt their journeys and much of the population is expected to pause in its daily routine. In the evening, thousands are expected to gather at Trafalgar Square, scene last week of celebrations of London's Olympic bid victory, to pay their respects and thank those involved in the rescue operation and medical treatment. There will be readings and poems from Londoners and those in the transport and emergency services.
Survivors of the blasts, as well as Metropolitan Police officers investigating the attacks, will join in the silence. The capital's moment of reflection is being co-ordinated by the Greater London Authority in conjunction with the TUC and various religious groups.
Tony Blair is expected to mark the silence at Downing Street and Tessa Jowell, the Secretary of State for Culture, will be at the family support centre in the Royal Horticultural Halls in Westminster.
A spokesman for the BBC said the two-minute dedication would be screened on BBC1, BBC2, News 24 and the Parliament channel.Normal programming will continue afterwards.
Mr Livingstone said: "London will remember all of those who died last Thursday and show its defiance of those who try to change the character of our city through terror. At noon, millions of Londoners will observe two minutes' silence. Every bus in the city will stop, businesses will stop and I want everyone who can to come out of their workplaces and homes on to the streets of London to remember those who died and to show their complete defiance of the terrorists."
Mr Livingstone said the show of strength would demonstrate "that London will not be moved from our goal of building an open, tolerant, multiracial and multi-cultural society showing the world its future".
Take-offs and landings will be kept to the minimum during the two-minute period at Heathrow and Gatwick airports, and no trains will leave London's main stations during the remembrance. The Underground said that as many of its staff as possible would observe the silence. It said trains would continue to run, "but there will be announcements about the silence on the trains".
British Telecom said workers at its call centres would not make calls during the two-minute period. All schools which have not broken up for holidays are expected to observe the silence.
The Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor, will lead the midday ceremony in the Westminster Cathedral piazza, alongside the Lord Mayor of Westminster.
The rally is expected to turn into a symbolic protest as powerful as the one that brought two million people to the streets of Madrid last year after al-Qa'ida-inspired bombs killed almost 200 people.
Silent moments to mourn
* The BBC will screen the two-minute silent commemoration on BBC1, BBC2, News 24 and the Parliament Channel.
* At Heathrow and Gatwick airport, no flights will take off or land during the two minutes.
* Network Rail says no trains will depart from mainline London stations.
* The National Portrait Gallery and Natural History Museum will fall into silence.
* Shopping centres will come to a standstill.
* Lloyd's of London will ring its Lutine bell, as it did after the 11 September 2001 attacks on New York and Washington DC.
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