- The Arade dam is 31 miles (50km) from where the British toddler went missing in Praia da Luz 16 years ago.
- The operation is being led by German police looking for evidence to link her disappearance to Christian Brueckner.
Police officers have started digging near a reservoir in Portugal in a long-running investigation into the disappearance of Madeleine McCann.
The Arade dam is 31 miles (50km) from where the British toddler went missing in Praia da Luz 16 years ago.
The operation is being led by German police looking for evidence to link her disappearance to Christian Brueckner.
The 45-year-old German national was made a formal suspect, or an "arguido", by Portuguese prosecutors in 2022.
Brueckner is known to have visited the picturesque spot around the time Madeleine, who would now be 20, went missing.
Sniffer dogs were used in a search of the reservoir's banks, and police officers went out into the water on an inflatable boat.
Uniformed and plain-clothed officers spent a number of hours scouring the scrubland, using pickaxes and inspecting small rocks with rakes and spades.
Emergency services and officials from Portugal, Germany and the UK held briefings near blue police tents erected at the site.
Speaking to the German regional public broadcaster NDR, German state prosecutor Hans Christian Wolters said the authorities "have grounds to believe that we could find evidence there (in the reservoir)".
In a separate statement to German TV channel RTL, Mr Wolters said he could not "give any concrete information" on the clues on which the search operation is based.
"They are not tips that come from the accused... but you can imagine that we don`t start searching somewhere in Portugal on the off chance, but that there must be a good reason for it.
"We do have one, but I ask for your understanding that I cannot disclose it here at the moment for tactical reasons."
Around lunchtime, more than 20 officers were digging beside the reservoir. A number of bags have been taken away from the search area, although it is not known what is in them.
There are reportedly four areas of interest to be searched this week.
A short statement from the prosecutor's office in the German city of Braunschweig has confirmed the search but did not say why it was taking place.
The Metropolitan Police said its officers were in Portugal so they can inform Madeleine's family of any developments.
It is not the first time the reservoir has been searched as part of the investigation.
In 2008, Portuguese lawyer Marcos Aragao Correia paid specialist divers to check the waterway after an apparent tip-off from criminal contacts that Madeleine's body was in the water.
Madeleine, from Rothley, Leicestershire, was three years old when she went missing on 3 May 2007 while on holiday with her family in the Algarve resort of Praia da Luz.
Her parents, Kate and Gerry, had gone for dinner with a group of friends at a resort restaurant, leaving Madeleine and her younger brother and sister sleeping in their apartment 100 yards away.
The adults had devised a rota to check on all of the group's children during the evening. But when it was Kate's turn, she discovered Madeleine had gone.
Mr Wolters is treating Brueckner as the main suspect in the McCann case, although he has never been charged over Madeleine's disappearance and has denied any involvement.
The state prosecutor said a growing amount of evidence had connected Brueckner to the case, including his mobile phone records showing he was in the Praia de Luz area at the time of Madeleine's disappearance.
Brueckner is currently in prison in Germany for the rape of a 72-year-old woman in 2005 in the same area where Madeleine went missing.
He was living near the Praia da Luz resort when the McCann family was on holiday, and spent time in the area between 2000 and 2017.
The most recent search in Portugal in relation to Madeleine's disappearance was in 2014, when British police were given permission to examine scrubland near where she vanished.
Earlier this year three other charges, unrelated to the McCann case, were thrown out of court because Brueckner's lawyer argued that, because of Brueckner's last place of residence in Germany, prosecutors in another German region should be responsible.
State prosecutor Mr Wolters has launched an appeal against this and, in Tuesday's comments to German media, clarified that he is still the prosecutor in charge of the case.