Ken Livingstone has been meeting those helping victims of the 7 July bombings, in a visit which had to be postponed after a second wave of attacks.
Mr Livingstone's planned visit coincided with the second attacks
The mayor was meant to visit volunteers at the Family Assistance Centre near Victoria last Thursday - the day four more bombers targeted the Tube.
The centre has helped 65 people who were bereaved or injured, as well as traumatised bus and Tube passengers.
In the condolence book the mayor wrote: "thank you for all you have done".
On his rescheduled trip, Mr Livingstone met the volunteers staffing the centre, which has taken 609 calls to its helpline and 420 visitors in total.
24-hour support line
He also sought assurances from the government that victims would not pay tax on grants from the £6m relief fund, raised by donations from the public, firms and charities over the past two weeks.
And he said the family of Jean Charles de Menezes, the man who was shot dead by police after being mistaken for a suicide bomber, would also be eligible for a grant.
"He is as much a victim as all the people we are seeing here [at the centre]," said Mr Livingstone.
The Treasury has said there is "absolutely no question" of people paying tax on their grants, the first of which were given out last week.
Bereaved relatives were given £5,000 while grants of £3,000 were given to those hospitalised in the attacks, they will be topped up at a later date.
Mr Livingstone toured the family assistance centre, near Victoria in central London, meeting volunteers, on Tuesday.
They offer emotional and practical support to those directly affected by the attacks on 7 July, when four suicide bombers killed 52 people and injured more than 700 others.
The centre took a couple of days to get up and running, but to avoid delay in the future, the mayor's office will be in charge of setting it up.
Its furniture will eventually be put in storage and brought out if another centre needs to be established if there is another attack, or a major accident.