Silence descended on cities, towns and villages around the world yesterday as people remembered the horrors of September 11, one year on. Nowhere was the stillness more poignant than at Ground Zero where thousands gathered, many grieving for lost loved ones, to mark the exact moment when the first jet struck the World Trade Centre.

Only the occasional sob could be heard at 8.46am (1.46pm London time) as people surveyed the giant void where, 12 months ago, New York's two tallest towers stood. Some held up pictures of those who died.

In London, a torn and tattered Union flag pulled from the rubble of Ground Zero was the focus of a moving ceremony. The flag had been brought to the UK by New York police lieutenant Frank Dwyer and was taken to St Paul's Cathedral, where relatives of many of the 67 British victims of the terror attacks, attended a memorial service timed to coincide with the Ground Zero commemoration.

"This flag, torn and tattered, still may be flown and is a rich symbol,'' said Mr Dwyer. "It is a symbol of endurance and the strength of the British people and the pain and agony they went through that day - that consecrated day.

"This flag belongs back in this land."

More than 3,000 petals, one for each victim, rained gently down from the Whispering Gallery as the service, attended by the Prime Minister, the Prince of Wales and Prince Harry, drew to a silent close.

The ceremony was one of many events around the world marking the anniversary of the attacks on the World Trade Centre and the Pentagon in Washington, and the loss of Flight 93 in Pennsylvania, when brave passengers thwarted a fourth hijack.

In total, 3,042 people lost their lives in the attacks. At Ground Zero, the names of the 2,801 who died there when the Twin Towers collapsed were read out one by one. About 24,000 of their relatives were allowed into The Pit for an emotionally-charged ceremony.

"They were us - our neighbours, our countrymen and our friends," said mayor Michael Bloomberg.

Thousands of people lined streets which were once dominated by the Twin Towers, and most of the city's fire stations were mobbed by thankful crowds. As night fell, candlelight vigils lit up a city that is usually all neon.

Every British police force had sent at least one officer to the ceremony out of respect for the New York cops and firefighters who died. The "bobbies", as the Americans call them, formed a guard of honour along Church Street before entering The Pit for the service.

"I wanted to come for the experience and to pay my respects," said North Yorkshire's representative Sergeant David Horne from Harrogate. "All the police we have come into contact with are extremely friendly and appreciate the Brits being here."

Yesterday morning, President George W Bush visited the sites in Washington and Shanksville, Pennsylvania, talking to relatives.

Standing beside a rebuilt Pentagon, Mr Bush mourned on the murder of so many innocents a year ago - and in their memory committed the US military to victory in the war the terrorists began.

"Today we remember each life. We rededicate this proud symbol and we renew our commitment to win the war that began here," a sombre president said, as he led America's anniversary observances.

"Though they died in tragedy, they did not die in vain. Their loss has moved a nation to action in a cause to defend other innocent lives across the world," he said.

Gesturing with his right hand clenched into a fist, the commander in chief added: "As long as terrorists and dictators plot against our lives and our liberty they will be opposed by the United States Army, Navy, Coast Guard, Air Force and Marines!"

He was due to arrive at Ground Zero late last night to lay a wreath, and was expected to make a television address to the American nation in the early hours of this morning.

He is also expected to make a significant address to the United Nations in New York today about the next phase of the war against terrorism.

America remained on "orange alert", the second highest level of danger, as the FBI said "credible sources" informed it of possible terrorist attacks. For US properties in the Middle East, the alert was raised to red, the highest possible level.

The alert demanded that Vice-President Dick Cheney spent yesterday in an "undisclosed location" away from President Bush.

The only real panic came when a 41-storey office block in Columbus, Ohio, was evacuated when a sniffer dog detected explosives.

But, although New Yorkers went about their business as normally as possible yesterday, there was a growing pessimism on the streets.

"The further away from 9/11 we get, people think that this will probably come again in some form," said Randy Smith, a Desert Storm veteran, who had driven nine hours from Ohio to attend the service at Ground Zero.