Parents of children who have disappeared, including those of Madeleine McCann, 4, are marking the EU's Missing Children's Day.
John McCann said the family would not stop until Madeleine was found
John McCann, Madeleine's uncle, on a visit to UK charity Missing People, urged families in a similar position to remain hopeful.
The key was to realise that there was a channel of support, said Mr McCann.
The charity said that since Madeleine's abduction on 3 May there had been 1,200 reports of missing young people.
The aim of the day, instigated by the European Union, is to support parents like the McCanns.
In the UK, Missing People, previously known as the National Missing Persons Helpline, chose the day to relaunch under its new name and logo.
It also announced what it said was the first UK direct mailing appeal to help find missing children.
And the charity launched an official yellow Missing People ribbon to symbolise support for all missing people.
People have been urged to wear yellow ribbons to mark their support for the McCanns since Madeleine was snatched from their holiday apartment in Praia Da Luz.
Madeleine's picture was projected on to Marble Arch in central London
John McCann visited the charity's offices in London to highlight its work and to offer support to other families whose children had disappeared.
He said: "I'm sure that you all can relate to the horrible feeling in the pit of your stomach and the complete turmoil that hits us.
"The initial waves of sickness and mental upset was completely overwhelming. None of us was able to think clearly."
He added: "For all families that are coping with a disappearance, your pain will be like ours and some of them will have carried it for longer than we have.
"What I want to do is show that you can remain hopeful. The key part is realising that there is a channel of support and that is where the charity Missing People comes in."
Mr McCann said the family was in it for the long haul and would not stop until Madeleine was found.
Missing People is appealing for help finding Carmel Fenech, 16
He joined Paul Tuohy, chief executive of Missing People, as he re-launched the charity.
"We are re-launching at a peculiarly ironic time - when the level of interest in missing people has perhaps never been higher, when 'missing' as a social issue is on the lips of politicians, radio and TV presenters, newspaper editors, and men, women and young people the length and breadth of the country."
Mr Tuohy also announced a direct mailing appeal for a missing child, which will be delivered to half a million homes on Friday.
It carries an appeal for a girl named Carmel Fenech who was 16 when she disappeared from Crawley, West Sussex, on May 23, 1998.
According to Home Office estimates, 210,000 people are reported missing each year in the UK, around two-thirds of whom are under the age of 18.
The EU Justice Commissioner marked the day with a plea not to forget the McCanns' plight.
Franco Frattini said: "The public support shown throughout Europe to the parents of Madeleine McCann has illustrated European citizens' solidarity with the families of missing children and the importance they attach to ensuring a safe and secure environment for our children."
All EU staff in Brussels were urged to wear forget-me-not (myosotis) flowers in support of the European Federation for Missing and Sexually Exploited Children, the organisers of the event.