The campaign, created by Nitro, involves an image of the words "1 London" highlighted in red within the phrase "7 million Londoners", which is written in black.
According to a statement from the Greater London Authority, all those involved in media and advertising are providing their services for free, including Clear Channel, Adshel, Maiden, Viacom, Bay Media, Primesight, Pawson Media, Odessa and Van Wagner.
Bruce Crouch, Nitro creative director, said: "We just wanted to do something positive in the wake of the bombings. We hope the campaign makes a contribution to helping encourage Londoners' unity and resolve."
The campaign is being placed in local papers as well as poster sites and the London Underground, which bore the brunt of the devastation during the 7/7 Islamist terrorist attacks and was the scene of a series of failed assaults on July 21.
EDF Energy, which is a sponsor of the London 2012 Olympic bid, is providing lamppost banners across London to promote the campaign.
Vincent de Rivax, EDF Energy's chief executive said: "I am delighted to support this campaign, which shows we stand united in difficult times as well as in celebration."
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