Transport for London (TfL) said services had resumed across the entire network in a "symbolic" tribute to those who died one month ago.
Maintenance staff and repair crews worked round the clock in often horrifying conditions to restore the Underground network, and this morning reopened two suspended sections of the Piccadilly line - from Uxbridge to Rayners Lane and from Hyde Park Corner to Arnos Grove.
Services on the Circle line - which is still being repaired following the Edgware Road blast - were also due to resume during peak hours this morning.
London Underground service director Howard Collins, who led the recovery operation, said: "It's important to all of us and important for this city that we have the entire network up and running this morning.
"The Circle line is the last element, and we hope that will be running, at least at 8.50amn (the time of the blasts), as a symbolic tribute to those who died."
Many Tube workers who helped rescue the injured and recover the bodies of the dead following the three devastating Underground bomb blasts on July 7, were back at work today.
The explosions, which were followed by a fatal attack on a double decker bus, were marked with a one-minute silence as the first Piccadilly line train passed through King's Cross to Russell Square.
Graphic photographs reproduced by an American TV station showed the horrific damage caused on the line when Germaine Lindsay, 19, detonated his bomb between the two stations, killing 26 people and himself.
Rescue and recovery teams braved almost unbearable conditions to retrieve the dead and carry out repairs to the deep-lying track, working 70ft below street level in heat of up to 60 degrees centigrade.
Dust, rats and asbestos added further complications, but a team of about 60 Tube line engineers worked first to assist police, and in the last few days, to repair the line.
The site, which was described as a "piece of hell", was covered in a thick layer of dust following the blast.
Mr Collins said it was a traumatic sight for workers who went down the tunnel to help in the aftermath of the attack.
"You could see the footprints of people in the dust, which was like black snow," he said. "Staff could actually see where people had been rescued, and led away through the tunnel and into the lifts.
"It has been difficult for the staff, but some of them have thrown themselves back into their work as a way of coping.
"It is easier for the station staff, as they are a team, a kind of family. For the drivers, it's been a lot harder, they have so much more time on their own, just thinking about what happened."
The first Piccadilly line service, which arrived at King's Cross at 5.37am, and at Russell Square 60 seconds later, was held on the platform for one minute, as staff and the few early morning customers held a silent tribute.
London Underground (LU) managing director Tim O'Toole was on board the service as it passed through last month's scene of devastation, and workers watched it from the platform.
At Russell Square, which was used as a temporary mortuary following the attack, a further tribute was spelled out on an electronic message board in the station forecourt. It read: "In memory of July 7 2005."
One of the few passengers to emerge from the line this morning said he was determined to return to his own routine.
Medical researcher Kanesalingam Suthaharan, 37, said: "I'm just glad things are getting back to normality.
"There was never any case of me feeling strange about things this morning, it's just time to get back on the Tube and get on with things."
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