9/11 anniversary: Charles and PM at remembrance event
The Prince of Wales and Prime Minister David Cameron have joined relatives of the 67 Britons who died 10 years ago in the 9/11 attacks on the US at a remembrance ceremony in London.
Wreaths were laid at the event in the September 11 Memorial Garden near the US embassy in Grosvenor Square.
Earlier, families of some of the UK victims attended services at Grosvenor Chapel and St Paul's Cathedral.
Services have also taken place in Glasgow, Birmingham and Belfast.
The Grosvenor Square event was organised by the September 11 UK Families Support Group, which set up the memorial garden in 2003.
Family members read the name of their loved ones and laid a white rose at the memorial, a small pavilion with three bronze plaques containing the names of the deceased Britons.
The Duchess of Cornwall, London mayor Boris Johnson, Deputy Prime Minister Nick Clegg, Labour leader Ed Miliband and the US ambassador to the UK Louis Susman were also in attendance.
The US flag on the embassy flew at half mast as Mr Susman led a minute's silence and Dame Judi Dench gave a reading.
Mr Susman said: "For those here remembering someone close, torn from you in the most brutal way, deprived of some of life's most treasured moments, 9/11 of course has touched your life immeasurably.
"Yet our societies are still strong, our political institutions and justice systems still function..."
Prince Charles, Mr Cameron and Mr Susman laid wreaths at the memorial, which also provided the backdrop for an evening concert.
In his speech Prince Charles spoke of the "continuing, awful agony" suffered by the bereaved but said he hoped for the "healing the world so desperately needs".
Nearly 3,000 people died when four hijacked airliners were crashed into the World Trade Center in New York, the Pentagon and a Pennsylvania field.
Kevin Dennis, who was working as a stockbroker for Cantor Fitzgerald on the 101st floor of the WTC's North Tower, was one of the Britons killed in the attacks.
Ahead of the event, his mother, Patricia Bingley, told the BBC: "I just want to keep his memory alive, and with Grosvenor Square, with the anniversary, I lay a rose for him and then I call his name and that's his day."
'Courage and dignity'
The UK commemorations began just after 08:00 BST at the nearby Grosvenor Chapel.
It was an intimate and low-key service, said BBC News correspondent Andy Moore.
The centrepiece of the service was the lighting of a single candle by deputy American ambassador Barbara Stephenson, in memory of those who died.
Canon Jim Rosenthal, who led the service said the attacks knew "no race, creed, gender, age or status" but remembrance "gives us the opportunity to take hold of the past and transform it to reach out with grace, understanding and healing".
US cleric Dr Courtney Cowart, was nearly buried alive in the WTC rubble, recalled the "extreme acts of love drove the darkness out".
The families of the dead Britons were later joined by about 2,000 representatives from the fire, police and ambulance services at St Paul's Cathedral.
Members of the UK's Firefighters Memorial Trust laid a wreath in memory of the 343 members of New York's fire department killed while responding to 9/11.
Sir Ken Knight, from the trust, told the BBC: "The fire service around the world is a very large family and we face the same hazards and the same risks and the same challenges."
At 13:46 BST, a minute's silence was held outside the US embassy in London to mark the time American Airlines Flight 11 crashed into the WTC.
About 60 protesters, including members from the Muslims Against Crusades group, were present in Grosvenor Square and set fire to a US flag during the silence.
A similarly sized group of English Defence League protesters gathered in response to the demonstration, while another group held up placards reading Muslims Against Extremism.
Scotland Yard said four people were arrested for public order offences.
Later, two men were stabbed at a pub in central London where the EDL says its members gathered after the demonstration.
The two were taken to hospital but their conditions are unknown.
The Met Police said there have been 10 arrests after an attack at the Tyburn Pub on Edgware Road.
A service was held at Westminster Abbey and other ceremonies took place in Plymouth and at Birmingham Cathedral, Truro Cathedral and Exeter Cathedral.
In Cornwall, there was a minute's silence at the Rick Rescorla memorial in Hayle. Security manager Mr Rescorla, 62, who grew up in the town, died after leading more than 2,000 people to safety on 9/11.
Meanwhile, faith leaders and politicians took part in a peace walk in Edinburgh, and the first minister attended a service at St Nicholas Kirk in Aberdeen. Wales First Minister Carwyn Jones said 9/11 saw "the best and worst of humanity" and sent his condolences to the US ambassador to the UK.
In the US, commemorations were held at the British embassy in Washington, and in New York the consul general attended a memorial concert at the British Garden in Hanover Square where the 67 UK victims are commemorated.
The Archbishop of Canterbury, Dr Rowan Williams, who was in Manhattan just two blocks away from the World Trade Center when the South Tower collapsed, told BBC Radio 2's Sunday Programme: "I think that was one of the deep moments, that everybody just wanted to pray together. And we did that as we heard the unforgettable noise of the first tower coming down."
Foreign Secretary William Hague paid tribute to the "courage and dignity of the American people" and the victims of other attacks including the London bombings of 2005.
In a statement, Mr Hague added: "So while we remember the victims of 9/11, stand firm with our allies and remain tirelessly vigilant against future threats, we also face the future with confidence in our values and faith in human nature".
Writing in the Sunday Mirror, US President Barack Obama said 9/11 "was not only an attack on the United States, it was an attack on the world and on the humanity and hopes that we share".