Friday marks 12 year since 52 people died and hundreds were injured when four suicide bombers attacked the city's transport system in 2005.
Sadiq Khan said Londoners "always pull together" in light of recent attacks in London Bridge, Westminster and Finsbury Park.
A wreath-laying ceremony took place earlier in honour of the victims.
"As well as grieving those we have lost, we also pay tribute to the heroic efforts of our emergency services and transport colleagues on that darkest of days", Mr Khan said
"Our firefighters, paramedics, police and public transport staff fought to save lives in the most horrific and harrowing of circumstances.
"The way our city stood together in the aftermath of the 7/7 attacks inspired the world.
"When Londoners face adversity, we always pull together."
Suicide bombers Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19, brought horror to London on July 7 2005.
Travelling from Luton, they took a train to King's Cross in London, hugged and separated to carry out the atrocities.
Within three minutes of 08:50 BST, Tanweer detonated his bomb at Aldgate, Khan set his device off at Edgware Road and Lindsay blew himself up between King's Cross and Russell Square.
Hussain detonated his device on a bus at Tavistock Square at 09:47 BST.

The 11-year-old's recording of If Every Day Was Christmas received its first radio play on Wednesday before being made available online.
Profits from the song's sales will go to Global's Make Some Noise charity, which supports disadvantaged young people in the UK.
Beckham is being managed by Scooter Braun - who also signed Justin Bieber.
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Bieber was 13 years old when he released his first single One Time in 2009.
Earlier this week, the young Beckham launched his own Instagram account, where he has been posting images to promote the single.
He also used the page to upload videos of himself singing. In one clip, he is seen performing a cover of Hopeful by Twista featuring Faith Evans.
Beckham Jr also uploaded a cover of When I'm Gone - originally recorded in 1931 by the Carter Family but made famous more recently by Anna Kendrick, who performed it in the film Pitch Perfect.
Speaking to Capital presenter Dave Berry about his debut single, the 11-year-old said it combined "two of my favourite things - singing and Christmas".
His father David, who was in the room when he was being interviewed, said he was "really proud" of his son for venturing into music, adding: "He's having fun, he's enjoying it."
But the move was criticised by Good Morning Britain host Piers Morgan, who said: "Why don't you go to school, mate, because you're not famous, your parents are famous... stop putting these kids into the public domain while simultaneously saying privacy."
Scooter Braun hit back on Twitter, saying of Cruz: "He is an 11-year-old who wanted to make a song to help other kids. Once again Piers shows why he is a jackass and why his [US] show was cancelled.
"Some people in this world make a living out of finding the worst in every situation. It means they have a career... not class. Just a career."
Last week, Beckham's older brother Brooklyn announced he is to release his own photography book.

Royston Jones, 39, from Brynmawr, was jailed for 15 years for the attempted murder of his wife Claire at the intensive care unit where she worked.
Plans for security staff to be on site at the hospital were also accepted.
Aneurin Bevan University Health Board said work to implement the measures was "underway".
The investment will enable CCTV to be expanded and monitored while security doors and access points will be upgraded, a health board spokesman said.
A second phase of funding was also approved for two security staff. They will initially be on duty overnight and at weekends.
The victims of the 7 July attacks will "never be forgotten", the mayor of London has said.
[NEXT_CONCEPT]
Cruz Beckham, the son of David and Victoria Beckham, has released his debut single.
[NEXT_CONCEPT]
A £120,000 investment has been approved to boost security at Abergavenny's Nevill Hall Hospital after a nurse was attacked on site.