Femi Oyebode, head of psychiatry at Birmingham University, said there were three stages in a person's response to a trauma such as a bomb blast or a serious accident.
The first stage was an acute stress reaction which anyone could suffer if they were exposed to exceptional stress. The next phase was acute stress disorder where symptoms included feelings of anxiety and palpitations, and the most severe reaction was post-traumatic stress disorder. This caused long-term damage which could last for months or even years.
Professor Oyebode said: "For the vast majority, the feelings of anxiety and lack of control, especially for those people who may have been stuck on a Tube, will subside within a day or two. But there will be a small group who will suffer acute stress and any reminders of what happened will make them burst into tears or feel angry and despairing."
It is expected that the victims will be offered counselling. But some experts warned that counselling was not the solution for everyone and could even aggravate people's problems. Leslie Carrick-Smith, a chartered forensic psychologist and member of the British Psychological Society, said that getting people to talk about the horror of what they saw or experienced could actually do a lot of harm.
He said: "We do not understand yet how fully people may react in these situations. People will be utterly numbed because if you're fighting a war then you're prepared for the worst to happen but in this case people were powerless to do anything and it's happened at such a personal level. In some cases the 'cup of tea approach' and for families to offer comfort to each other will be the best way forward."
James Thompson, a trauma psychologist at University College London, said some people suffering post-traumatic stress took up to five years before they saw a doctor. Dr Thompson said he had been involved in the aftermath of the King's Cross fire in 1987 and patients were turning up with trauma symptoms many years afterwards.
But he said at the moment people would still be coming to terms with what had happened. "Londoners, although we have always been aware that this was a possibility, are now having to process and understand that it has really happened."
Dr Thompson said it was likely people would watch hours of television - probably more than they should - to try to understand what had happened. "In subsequent days you will see major changes in behaviour. People who don't have to go into London probably will not.
"That will probably mean fewer shoppers but the number of commuters will probably stay the same. Anxiety levels will be up and people might want to avoid deeper Tube lines.
"There will also be more concentration on abandoned bags and suspicious behaviour and people will probably be less scared of saying something if they have concerns."