Police have removed the body of a young boy from woodland as they continue to question the mother of three-year-old Mikaeel Kular.
The discovery was made near her former home in Kirkcaldy, Fife, and announced by police just after 1am.
A post-mortem examination is understood to be taking place in Edinburgh in the coming hours.
There were gasps from people outside Police Scotland's Edinburgh HQ as Assistant Chief Constable Malcolm Graham announced the body had been found shortly before midnight.
Police said they had detained one person - understood to be Mikaeel's 33-year-old mother Rosdeep - in connection with his disappearance.
:: Live updates as the search for Mikaeel Kular is called off
ACC Graham said: "The investigation into the disappearance of Mikaeel Kular has been wide-ranging and fast moving.
Forensics officers have been examining an area of woodland
"As a result of enquiries the body of a young child was recovered in Fife just before midnight.
"We strongly believe this to be the body of Mikaeel.
"A person has been detained in connection with the recovery of the body and members of Mikaeel's family have been informed of the recovery."
The discovery is understood to have been made on land surrounding a house in Kirkcaldy, Fife, which is owned by Mikaeel's aunt Pandeep Kular.
The body was removed from the scene at around 3pm on Saturday afternoon.
There was shock when ACC Malcolm Graham announced a body had been found
Sky News Scotland Correspondent James Matthews said: "There is no suggestion that police want to speak to her (the aunt) about any of this at this stage."
A neighbour told Sky News that Rosdeep Kular visited the house alone, by car, last week.
The family lived there for a time before moving to Edinburgh and neighbours remember Mikaeel and his twin sister playing on the steps.
A memorial service organised on Facebook will take place at 7pm today at St Andrew's church in Muirhouse, Edinburgh.
Mikaeel was reported missing from his home in the Scottish capital on Thursday morning.
Hundreds of police officers from across the UK joined the search and lines of volunteers swept the local area, searching for clues about his disappearance.
Police were told that Mikaeel had not been seen since his mother put him to bed on Wednesday night.
The body was found around 30 miles from Mikaeel's family home in Edinburgh
On Friday evening ACC Graham revealed the boy had not attended nursery since before Christmas and said there were "grave concerns" for his safety.
Sky News Crime Correspondent Martin Brunt in Edinburgh said: "We know that according to the police, a person was detained last night.
"The police haven't named that person detained, but we understand from our sources that it's Mikaeel's mother Rosdeep, known as Rosie.
"Under Scottish law, she can be detained initially for 12 hours during questioning and then on the say so of a senior officer can be held for another 12 hours.
"At the end of that period police must decide whether to release her, charge her or formally arrest her."
Police analyst Graham Wettone told Sky News: "(His mother) will be questioned as police try to establish what has taken place."
Neighbours have been leaving flowers and toys near the family home
Police Scotland officers have cordoned off the road and been carrying out searches in woodland around Pandeep Kular's Kirkcaldy house.
There was sadness and anger among families living near Rosdeep Kular's home in Ferry Gait, Edinburgh - some 30 miles away across the Firth of Forth - as they woke up to news of the grim discovery.
MSP Colin Keir, who joined the search for Mikaeel, was woken at 5am to be told the news.
He told Sky News: "It's just devastating and I think the community will take it personally.
"There was an unprecedented outpouring from the community in the hunt for Mikaeel.
"The community will be devastated, angry and looking for answers as to why this has happened."
Hundreds of people volunteered to help with the search for Mikaeel
Jackie Brownlee, 29, a mother-of-two who lives opposite the family home, said that local people will support each other as they try to come to terms with what has happened.
She said: "Everyone was hoping and now that hope's gone - we still can't believe it. But I think the community will come together because we have to."
Her son Darren, 11, added: "He was such a cute lad, always running around the street. I can't believe it's happened in my street - I'm really scared now."
Scotland's First Minister Alex Salmond said: "This is a tragic development in this harrowing story and the news that we have all been dreading.
"The hearts of everyone across Scotland and beyond have been touched by the events of recent days."
Police Scotland are also investigating an offensive Twitter comment relating to the case, which originated in the Greater Manchester area and was reported by an Edinburgh resident.
Denise Fergus, the mother of James Bulger, the two-year-old who was murdered in Liverpool in 1993, tweeted her condolences.
She said on Twitter: "My heart is aching for that poor boy Mikaeel Kular brings it all back so sad."
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