In London, workers poured out of their offices to line the streets while taxis and buses came to a halt. Planes at airports switched off their engines and delayed take-offs.

Hundreds gathered at King's Cross station, site of one of the blasts detonated by a group of young British Muslims of Pakistani ethnic origin who lived in northern England.

Prime Minister Tony Blair, who on Wednesday said he would look urgently at new measures to tackle extremism, marked the silence in the garden of his Downing Street office, while Queen Elizabeth observed it at Buckingham Palace.

Golfers at the 134th British Open championship stood quietly on the fairways and greens of the St Andrews course in Scotland.

Tributes were also paid in Madrid and Bali -- both targeted by bombers from the Islamist al-Qaeda network in the past -- and in cities across Europe. The Pope, on holiday in the Italian Alps, prayed for peace.

Forty miles northwest of London, police continued to search a house in the market town of Aylesbury for clues to who may have plotted the bombings -- the first suicide attacks in western Europe.

British police have identified the man thought to be the mastermind behind last week's bombings in London.

The British-born man in his 30s, of Pakistani origin, arrived at a British port last month and left the country again the day before Thursday's attacks, The Times newspaper reported.

The four suspected suicide bombers, three of whom have been identified by newspapers, were also Britons of Pakistani origin.

According to The Times, security sources believe the mastermind was involved in previous terror operations and has links with followers of Osama bin Laden's al-Qaeda extremist group in the United States.

He is thought to have visited the bombers in their home city of Leeds, northern England, and identified targets on the London Underground rail subway system where three of the four bombs exploded, the paper said.

Security sources also said he was likely to have trained the recruits in how to trigger their rucksack-carried bombs, three of which went off almost simultaneously at around 8:50 am (0750 GMT), at the peak of last Thursday's morning rush hour.

Police have reportedly switched their investigation towards seeking the mastermind of the bombings since learning that the four bombers killed themselves in the attacks.

They raided the house on Wednesday night but made no arrests and found no explosives.

Officers were also questioning a 29-year-old man arrested on Tuesday on suspicion of the "commission, instigation or preparation of acts of terrorism."

He was detained in Leeds, the home town of at least three of the bombers, details of whom are still emerging.

One was 22-year-old Shehzad Tanweer, a keen cricketer who helped out at his father's fish and chip shop. Friends and family said he was fanatical about sport but not about politics or religion.

The other two have been named as 18-year-old Hasib Mir Hussain and Mohammed Sadique Khan, 30, a husband and father who worked as a teaching assistant looking after disabled children.

"He was 100 percent committed to the school and to the children and worked extremely hard with children and families," said his former headteacher Sarah Balfour. "We are all extremely shocked and find it hard to understand."

The fourth bomber has not been named.

Britain's newspapers were dominated by details of the men.

"The boy who grew up to bomb the No 30 bus," was the headline in The Independent next to a photo of Hussain as a 10-year-old schoolboy.

He blew himself up on a double-decker bus in central London, an hour after the other three bombers struck in quick succession at three London Underground stations.

The Guardian printed a photograph of Shahara Islam, a 20-year-old who was one of Hussain's victims, and contrasted her story as a young British Muslim with his.

She was one of 22 victims of the bombs to be identified and 11 who have been named. Coroners are still trying to formally identify the other 30 victims and police say they expect the death toll to rise.

Several newspapers identified more suspects who they said police were hunting.

One was described as an Egyptian chemistry student at Leeds University who had lived in the same area of the city as the bombers but who had disappeared days before the attack.

Another was described as a British-born al-Qaeda operative who arrived in the country by sea three weeks ago and left again hours before Thursday's blasts.

According to The Times newspaper yesterday, police are also seeking a possible fifth member of the bombers' terror cell, who was also seen at Luton station, north of London, from where the attackers travelled into the capital.

The man, also believed to be of Pakistani origin, could still be at large in London, the paper said.

(Reuters, AFP)