Thousands of people have stood side-by-side on the streets of London for a two-minute silence in memory of the victims of last week's bombings.
Cars and buses stopped during the silence in London
People left their offices and traffic came to a halt as silence fell across the city at 1200 BST.
Fifty-four people died and 700 were injured in the blasts. Three of the four bombers are included in the dead.
Police sources believe the fourth bomber was Jamaican-born Lindsey Germaine, who lived in Buckinghamshire.
He is understood to have lived in the house in Aylesbury which was raided by forensic experts on Wednesday night.
For two minutes from 1200 BST, train stations and airports fell silent, buses pulled into the kerb and busy shopping streets became still, as people stopped what they were doing to observe the silence.
People stood in the streets with their heads bowed. A vigil was held in Trafalgar Square at 1800 BST.
The Tube kept running, but passengers were advised the silence was taking place and encouraged to participate. Many London Underground staff wore black armbands.
The Queen marked the ceremony at Buckingham Palace, Tony Blair was at Downing Street and London Mayor Ken Livingstone was with faith leaders and members of the city's Olympic bid team at Trafalgar Square.
Mr Livingstone told the BBC after the silence: "This city has survived the past week because we didn't turn on each other, which is what the bombers wanted. We supported each other."
Wreaths were laid at the bombing sites as hundreds stood at each to pay their respects.
People in towns and cities across the UK also observed the silence.
Ceremonies were also held at European Commission offices across all 25 EU member states. The EU Parliament also fell silent.
Bastille Day celebrations in Paris were temporarily halted.
On Thursday, Scotland Yard released images of bus bomber Hasib Mir Hussain, 18, one caught on CCTV as he went through Luton station, another on his driving licence.
Mohammad Sidique Khan(above): Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
Hasib Mir Hussain: Aged 18, lived Holbeck, Leeds. Reported missing on day of bombings. Said to have turned very religious two years ago. ID found in No 30 bus.
Shehzad Tanweer: Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast.
Fourth bomber: Understood to be Jamaican-born Lindsey Germaine, who lived in Buckinghamshire.
They appealed for any of the 80 people thought to be on the bus at the time of the explosion to contact police if they had not already done so.
Dep Asst Cmssr Peter Clarke also confirmed Shehzad Tanweer, 22, the bomber behind the explosion between Liverpool Street and Aldgate stations.
Mohammad Sidique Khan, 30, is suspected of causing the Edgware Road bombing.
Police are also hunting a fifth man who they believe masterminded the attacks. It is believed he left Britain shortly before the bombings.
A sixth man, an Egyptian chemistry student who has disappeared from his house in Leeds, is also being sought.
Police have been handing out leaflets to commuters in London in a bid to find more witnesses. Appeal posters have been set up around the capital.
In Leeds, a controlled explosion was carried out in the Beeston area after police cordoned off several streets and evacuated residents.
They have been told by police to take enough clothes with them for three days.
Hussain was caught on CCTV at Luton station
Earlier, police had lifted a cordon around another neighbourhood after it was found that a suspect substance in one house was not hazardous.
Fourteen of those killed have been formally identified, although police have yet to release the names of three.
Fifty of the injured remain in hospital, health officials said.
Before the two-minute silence, the driver of the number 30 bus targeted in last week's bombings, George Psaradakis, read a message of sympathy to the families of victims.
FAMILY ASSISTANCE CENTRE
Offers support for all those affected by London bombs
Latest information and investigation updates available
Address: Lindley Hall, 80 Vincent Square, SW1P 2PE
24 hr line: 0845 054 7444
"In today's silence we remember them. With quiet dignity and respect we show our deep contempt for those who planted the bombs and those who masterminded them.
"As we stand together in silence, let us send a message to the terrorists - you will not defeat us and you will not break us."
Later thousands are expected to gather at Trafalgar Square to pay their respects and thank rescue and medical staff.
Londoners and members of the transport and emergency services will recite readings and poems.
A relative of one of the suspects was arrested on Tuesday. Detectives have been given a warrant to hold the 29-year-old until Saturday on suspicion of commissioning, instigating or preparing acts of terrorism.