At a 24-hour assistance centre, specially trained officers helped people still searching for their loved ones, while at Heathrow Airport officers waited for families flying in from abroad.

Some cared for people at city morgues, and others were sent to homes to handle the sensitive and time-consuming task of standing by those who have almost certainly lost a family member.

Samples of fingerprints and DNA, including from toothbrushes and hairbrushes, are being discreetly taken to help with the identification process.

The Thursday morning rush hour attacks on London's public transport network killed at least 49 people and injured around 700. At least 25 others are thought to be missing.

With no bodies identified, their families are in anguish.

"I just don't know what to do. I'm going crazy," said Gous Ali, whose girlfriend Neetu Jain has vanished since boarding a downtown bus to go to work.

"It is very harrowing. The police have just been to see me to take a sample of my DNA," said Pawel Iskrzynski, whose sister Ania Brandt also went missing.

"I hope it is to prove that Ania is still alive and lying injured in hospital," he said.

"It's the wait that is the worst," said the "emotionally drained" sister of missing hair stylist Phil Beer. "There is nothing we can do."

According to senior London police spokesman Brian Paddick, dozens of expert "liaison officers" have been sent to visit relatives and friends.

At the Queen Mother leisure centre, around 100 police and staff supported by helpers from the British Red Cross, Salvation Army and several local authorities were at work.

Outside, a tight security cordon guarded their privacy.

As they arrived, family and friends were greeted by a police family liaison officer trained to deal with people in distress.

They can then speak to other agencies, getting help with anything from counselling and psychological support to practicalities like housing and travel for relatives who have come from outside London.

Some stay for several hours, many are deeply distressed.

"The purpose of this is to bring things together so we can reduce the distress to families by having them come to one place where they can access all their needs," said Commander Steve Allen from the Metropolitan Police.

It is also about identifying bodies in the most subtle way possible.

"Identification is a matter for the coroner but one of the key functions of the family liaison officer is to access the information from the families to help to do that," he said.

Allen said that would include descriptions, identifying the missing person's travel patterns and in some cases obtaining DNA and fingerprints.

Staff at the centre included people who had helped in the aftermath of the tsunami disaster in Asia last Christmas eve, said a spokeswoman from the British Red Cross, Liz Page.

"The needs of people in these circumstances vary a lot. Some people may need someone to talk to for a while. Some may be extremely distressed," she said. "Our people are trained to hear some very difficult things."

After a visit to the centre, Britain's Culture Secretary Tessa Jowell said that about 33 families had come through in the first 24 hours that it had been open.

"If families are wondering where they should come, this is the place," she said. "It is a very important and excellent source of information gathering and advice for families."