Tourists who have visited the Algarve resort where Madeleine McCann was abducted are being asked to send their holiday photographs to British police.
Gerry McCann read some of the many messages from well wishers
They want anyone who was in the Praia da Luz area in the two weeks before the four-year-old disappeared on 3 May to send their photos to a new website.
Officers will cross-reference them with a database of UK paedophiles.
Madeleine's father Gerry has returned to Portugal to rejoin his wife and children after a brief trip to the UK.
Mr McCann arrived back in Praia Da Luz on Tuesday morning after his one-day visit home.
During his trip, Mr McCann visited the family's home village of Rothley, Leicestershire, where he saw the thousands of yellow ribbons, tributes, flowers and cuddly toys left by the public.
He spoke to some of the people who had gathered to show their support and tied his own yellow ribbon, which has become a symbol of hope for Madeleine.
As well as meeting organisers of the campaign to find her, it is believed his visit was also intended to enable the family to stay in Portugal for the foreseeable future.
Accompanied by aides carrying a suitcase and parcels, Mr McCann returned to be with his wife, Kate, who had remained at the family's Algarve holiday apartment.
Making their appeal for photographs, UK police said they wanted pictures that included strangers in the background, but not family or empty scenic shots.
Photos can be uploaded via a website - www.madeleine.ceopupload.com.
The photographs will be cross-referenced against a database of pictures of UK paedophiles and other criminals, with police able to check 1,000 an hour.
UK POLICE APPEAL
Photos with strangers in the background
Taken between 19 April and 3 May
Volunteer searchers to contact police on 0800 0961233
Any relevant information will then be passed on to the Portuguese police.
Jim Gamble, of the Child Exploitation and Online Protection (CEOP) Centre, said: "No matter how small or insignificant the information may seem to you, it could be the missing part of the jigsaw, so let us decide if it is important.
"We are looking for anyone who was at the Ocean Club Resort or surrounding area in the two weeks leading up to Madeleine's disappearance on 3 May, who have photographs that might help our work."
Detective Chief Superintendent Anne Harrison, of the National Policing Improvement Agency (NPIA), said it was possible Madeleine was still being hidden in Praia da Luz.
She also appealed for anyone who took a part in the initial search for her, before the Portuguese police arrived, to contact police on 0800 0961233.
Police have teamed up with a number of computer companies to deliver the technological power behind the website and the collection of photographs.
As well as uploading them, police say the public can take any photos they might think relevant to high street photography shop Jessops, which will convert traditional prints to digital images for free.
A number of UK law enforcement agencies are helping the Portuguese authorities, including Leicestershire Constabulary, the Association of Chief Police Officers, CEOP and the NPIA.
Portuguese police have also confirmed they are investigating claims of a possible sighting of Madeleine in Morocco.
Chief inspector Olegario Sousa said the clue was "being verified" and that the Moroccan authorities had been contacted.
At noon on Monday a one-minute silence was held in Portugal to show solidarity with Madeleine's family.
The silence was initiated by an anonymous chain e-mail circulated around the country at the weekend and was observed by tourists and locals.
Liverpool FC have joined the campaign to find Madeleine
Madeleine's mother Kate McCann observed the silence privately at the family's holiday apartment.
Senior detectives in Portimao, who are investigating Madeleine's disappearance, also stopped for one minute.
The latest public display of support has come from the Liverpool football team, who were photographed on Monday with a banner asking for information of Madeleine's whereabouts.
They were pictured as they left the UK for their Champions League final in Athens on Wednesday.
Portuguese people have also been asked to pray for Madeleine on Tuesday night in a nationwide e-mail from Carmelite nuns.