The 52 victims of the 7 July 2005 bombings were "murdered" in acts of "merciless savagery", the inquests into their deaths has heard.
It was revealed that the attacks may have been planned for the previous day.
Coroner Lady Justice Hallett requested a minute's silence before the hearings into the suicide attacks on three Tube trains and a bus in London began.
She will examine how each victim died and whether MI5 could have stopped 7/7. Many families want a public inquiry.
In addition to the 52 people killed, some 700 people were injured, many of them severely and permanently, when four al-Qaeda-backed suicide bombers, all British men, detonated their devices.
Hugo Keith QC, counsel to the inquests, told the hearing that a text message from ringleader Mohammad Sidique Khan suggested he had abandoned original plans to carry out the attacks 24 hours earlier.
He texted fellow bomber Shehzad Tanweer at 0435 on 6 July saying: "Having major problem. Cannot make time. Will ring you when I get it sorted. Wait at home."
The inquest heard that Khan visited Dewsbury Hospital with his wife, Hasina Patel, on 5 July because of complications with her pregnancy. She miscarried on the day of the attacks.
In his opening statement, Mr Keith told the court some questions about the atrocity may never be answered.
He said the bombs "detonated amongst the innocent and the unknowing, indiscriminately killing and maiming passengers".
"They had no regard to whether the victim was Christian, Muslim, a follower of any of our other great faiths, an adherent to none. They were just travelling on the London transport system," he said.
Mr Keith said bombers Mohammad Sidique Khan, Shehzad Tanweer, Germaine Lindsay and Hasib Hussain had unleashed an "unimaginably dreadful wave of horror" by detonating the devices.
"They were acts of merciless savagery which could only outline the sheer inhumanity of the perpetrators."
He added: "The essential nature of these acts was murder."
Later he displayed transcripts of emergency calls and logs to the London Underground network control centre on the morning of the attacks.
One log from 0851 read "explosion Liverpool street", "train hit tunnel wall" and "person under train?"
The court heard live telephone calls into the London Underground centre, which become increasingly serious.
The centre advised that they believed it was a power supply problem rather than a terrorist-related incident.
But then a station supervisor called describing a big bang, while the Edgware Road manager rang to say: "People are coming up with blackened faces and blood on their faces - something's gone bloody wrong down there."
The hearings at the Royal Courts of Justice in London are expected to last until at least March next year and will look into the precise details of the 2005 attacks.
Witnesses will include survivors of the attacks on underground trains near Aldgate, Edgware Road and Russell Square Tube stations, and on a double-decker bus in Tavistock Square, near King's Cross.
Members of the emergency services who tried to save lives will also give their accounts.
And evidence will be heard from the families of the deceased to try to "give a flavour of who they were, their personal qualities, their gifts and their plans", said Mr Keith.
Lady Justice Hallett, who will preside over the hearings without a jury, asked for the names of those who died to be read out before the minute's silence.
The hearings are also expected to be shown footage and pictures of the aftermath of the attacks that have never been seen before.
Lady Hallett said as much information as possible would be released to the hearings.
"I will balance carefully the needs of national security with relevance and fairness," she said.
She added that she had yet to decide "whether it is in my powers, and if so, if it is in the interests of justice to conduct any closed hearings".
In a ruling earlier this year, she said she would also look into the backgrounds of the bombers and what the security services knew about them.
Two years after the attacks, it emerged that MI5 had come across the ringleader and one of the other bombers during their investigations into another extremist cell.
Some families believe the security services and police had enough information to work out that Mohammad Sidique Khan, the ringleader, was a threat. Security officials insist they only had fragments of information and could not have predicted what happened.
There have been two official reports into the bombings by the Intelligence and Security Committee in Parliament, both of which said that MI5 should not be blamed.
But Graham Foulkes, father of 22-year-old David, who was killed at Edgware Road, said he and other families were angry that the security service was still attempting to keep information out of the public domain.
"By every kind of moral standard that you're brought up with, that's wrong," he said.
"You're told, if you make a mistake, you hold up your hands. My view is that their incompetence allowed Mohammad Sidique Khan to get through."
Many of the relatives of victims are represented in the inquests and will be able to question witnesses.
Julie Nicholson, whose daughter Jenny also died in the Edgware Road blast, said: "The inquest represents a juncture at which a very well-respected judge can ask those questions so that every aspect of the events can be scrutinised and analysed.
"So you go from having all these jigsaw pieces in a bag, shaken up, to having a clearer picture, a story."
She said the key question for her was whether anything could have been done to prevent Jenny's death.
"I think I would like, before I reach my grave, to put that to rest, to understand that possibly yes it could," she said.
The hearings had been delayed because of criminal investigations and questions over what the inquests should cover.
But Mr Keith said the earlier investigations had not focused on the victims, whereas these inquests would, as well as looking at the issue of preventability.
The inquests of the suicide bombers have been adjourned and will be held separately.