Police were last night investigating whether Madeleine McCann was watched by her abductors three days before she was snatched.

Detectives have discovered that the McCann family went to the town of Sagres, on the southern tip of the Algarve, on April 30.

A witness has told police that on the same day he saw a suspicious man following families and photographing children, including his daughter, a blonde girl "strikingly similar" to Madeleine.

As the footballer David Beckham last night added to the appeals for help to find Madeleine, missing for eight days, the police investigation intensified.

More than a week into their ordeal, Gerry and Kate McCann are today painfully celebrating Madeleine's fourth birthday in her absence. A church service is planned and locals will tie yellow ribbons to trees as a symbol of hope to bring Madeleine back.

Detectives are working on the theory that the abductors may have seen Madeleine in Sagres and tracked her back to the family apartment in Praia da Luz, 18 miles away.

A witness, Nuno Lorenco, said he saw a man following his family down the beach and taking "discreet" photographs. He said he challenged the man, who fled with

a blonde woman in a Renault Clio.

Police have asked the McCanns for all the photographs from the trip to Sagres, and the rest of their holiday, looking for suspects in the background.

Among other leads being followed are reports that a blonde woman was seen acting suspiciously near Madeleine's apartment hours before she disappeared. Mr and Mrs McCann have been reviewing their evidence with police in the past 48 hours.

Having spent the week issuing their own appeals, it is the first time police have informed them of their progress. Mr McCann spent 10 hours at the police station in Portimao, leaving at 3am yesterday.

In a statement on behalf of the family, he said yesterday: "We have now seen at first hand how hard the police are working in the search for Madeleine and their strong desire to find her.

"We are doing absolutely everything to assist the police with their investigation and we will leave no stone unturned in the search for our daughter, Madeleine."

In the past two days police have been reviewing statements and checking leads.

They are following more than 350 "suspicious sightings". These include three suspects in a car with British number plates, captured on CCTV at a nearby petrol station on the night Madeleine disappeared.

One witness said he saw them with a girl matching her description.

Police have now discovered that two of them had earlier been staying at an apartment in Burgau, the neighbouring resort to Praia da Luz.

They are described as being in their 40s. The woman had long blonde hair while the man was said to be tanned.

Both were claimed to have been "discreetly" wandering around in the area and are believed to have spoken English in a foreign accent.

David Beckham, the Real Madrid footballer, made a television appeal yesterday

asking for information.

He held up a missing person poster of Madeleine as he delivered the appeal. He said: "We need to do everything possible to help police."