The Prime Minister and Britain's most senior police officer were among the guests today at a memorial service to mark the 20th anniversary of the death of murdered teenager Stephen Lawrence.
The 18-year-old died when he was attacked by a gang of racists as he waited for a bus in Eltham, south east London on April 22 1993.
His mother Doreen was joined by friends, relatives and supporters of the charitable trust that she set up in her son's name at St Martin-in-the-Fields church near Trafalgar Square in central London.
Mrs Lawrence told the congregation: "I've always tried to look forward and to focus on the positive since Stephen's murder, and looked to see how I can make the lives of others better.
"The pain of losing someone never goes away, you just learn to live with it at cost.
"Over the years as a family we have mourned in our separate ways, not daring to speak out loud about our feelings. My pain is raw, and that of my children."
The first candle at the service was lit by Mrs Lawrence and her surviving son Stuart and dedicated to building hope in memory of Stephen.
ITV News' UK Editor Lucy Manning reports.
Guests included senior political figures, Metropolitan Police Commissioner Sir Bernard Hogan-Howe, singer Emeli Sande, and Duwayne Brooks - Stephen's friend who was with him on the night that he died.
Lawyers Michael Mansfield and Imran Khan who have represented the Lawrence family throughout their battle for justice were also present.
Earlier, Mr Cameron told BBC News: "A change obviously has taken place since that dreadful murder 20 years ago, a change in policing.
"But perhaps as important is the change in culture, of just not accepting racism in our country.
"We have made huge steps forward on that front, but there is always more that needs to be done."
Mayor of London Boris Johnson told the congregation: "On a dark street in Eltham 20 years ago there took place a crime that caused shock and pain, and disbelief, and outrage.
"And it was left to those who had suffered the greatest pain and loss, the family of Stephen Lawrence, to search for the truth and for justice."
But he said the success of the Olympics had shown the best of the diversity of London.
"The world was watching us, and they saw that incorrigible happiness and that unity in diversity", he said.
"Out of that dark night 20 years ago has come great hope, and it is in a spirit of hope for London, the greatest city on earth, that we remember Stephen Lawrence today", Mr Johnson added.
The music spanned traditional hymns, through to Gospel and singer Beverley Knight performed a powerful rendition of Fallen Soldier, which was especially written for Stephen Lawrence.