A 33-year-old woman will appear in court tomorrow after being arrested and charged in connection with the death of three-year-old Mikaeel Kular.
Police did not name the woman, although it has been widely reported that Rosdeep Kular, the mother of the missing Edinburgh boy, was being questioned by officers after a child's body was discovered in Fife, miles from his home.
The development was announced late last night by Police Scotland Assistant Chief Constable Malcolm Graham, who said: "During the course of today, the investigation into the disappearance of Mikaeel Kular has continued at pace.
"Following formal identification, I can now confirm that a 33-year-old woman has been arrested and charged in connection with Mikaeel's death.
"The woman is expected to appear at Edinburgh Sheriff Court on Monday."
A major police investigation was launched to trace Mikaeel after he was reported missing from his home in Edinburgh's Ferry Gait Crescent on Thursday.
More than 200 members of the public joined police and other agencies in their searches for the youngster in the Drylaw area of the capital on Friday.
Officers searching for the three-year-old found remains - now identified as those of Mikaeel - in Kirkcaldy, Fife, shortly before midnight that day.
Mr Graham expressed his thanks to all those who aided police in recent days.
More than 500 calls were made by the public to a dedicated phone line set up in the wake of Mikaeel's disappearance, he revealed.
He said: "I would like to again recognise the overwhelming public assistance that Police Scotland have received during the course of this very complex investigation. This includes his family and friends, with whom our thoughts remain.
"Over 200 civilian volunteers joined the search for Mikaeel within the Pennywell, Muirhouse and Silverknowes areas of Edinburgh during adverse weather conditions and cold temperatures.
"In addition, our dedicated number received in excess of 500 calls, with people wishing to provide information to help establish Mikaeel's whereabouts.
"I would like to extend my sincerest thanks to you all for your vital support.
"My thanks also go to the various external search agencies who provided essential resources, equipment and personnel during this enquiry. Police Scotland is immensely grateful for your contribution.
"Inquiries continue, led by the Major Investigation Team of Police Scotland, supported by a range of local and national specialist assets from across the country. I ask that anybody who has any information that they feel would assist the investigation contacts the police."
Mr Graham said he would be unable to answer further questions about the investigation.
Mikaeel's disappearance sparked a major hunt involving hundreds of police officers and local volunteers.
Yesterday afternoon his body was removed from a wooded area behind a property in Dunvegan Avenue, Kirkcaldy.
Pandeep Kular, 37, Mikaeel's aunt, sobbed as she told the Sunday People: "We're devastated. I'm with the kids."
She said she had not spoken with her sister, saying: "I'm not aware of anything. I'm just with my family in the house. That's all I can say. We've just lost him."
Dr Bangarpet Krishnaswamy, who is married to Mikaeel's grandmother, gave an emotional tribute to those who helped search for the little boy, the Sunday Mirror said.
He said: "I would like to thank all the public and the police for the search, which sadly turned out to be a futile."
Throughout the day, communities in north Edinburgh and Fife gathered to support each other and show how they had been touched by the recent events.
Floral tributes, soft toys and candles have been placed at a small park in the Drylaw area, round the corner from the flat where Mikaeel lived on Ferry Gait Crescent.
Residents of Dunvegan Avenue, around 25 miles away, spoke of their shock at seeing police activity on their street.
Michael Woods, 53, who lives nearby at Floors Place, said he had been following the developing police investigation but was surprised to find officers at the end of his street.
"First it was in Edinburgh, then they said Fife, then Kirkcaldy and now it's here almost on my doorstep. It's unsettling," he said.
"When I heard it was here I just about jumped off my chair."
In Edinburgh, around 500 people turned out last night for a memorial service at Muirhouse St Andrew's Church, less than a mile from the home Mikaeel shared with his mother, twin sister and three other siblings.
There was a large queue to get into the church well in advance of the service. The crowds ultimately filled the 300-capacity building and an extra overflow room.
Men, women and children of all ages attended the memorial gathering. Some very young children clutched soft toys while a handful of people held lanterns.
At the end of the gathering, all those who had attended exited carrying the remains of candles they had been holding during the service.
Some people hugged each other and a Chinese lantern floated in the sky above the chapel.
Attending the service was Akeel Umar. He said: "I think everyone in the community is very sad. Today is an opportunity for everyone to come together on an issue that touches everyone."
Nasim Azad added: "We are a community, we care. The fact that we're here today is because we care. It's as simple as that."
Also attending the service was Edinburgh Western MSP Colin Keir.
He said recent events had shattered the whole area, but also galvanised a sense of community spirit.
Meanwhile, Derbyshire Police said they had assisted colleagues at Police Scotland by arresting a man on suspicion of sending offensive tweets in relation to the disappearance of the toddler.
A force spokesman said: " Officers arrested the 26-year-old Heanor man this morning on suspicion of committing a racially aggravated public order offence.
"Members of the public in Edinburgh had complained about the content of some Twitter messages linked to the disappearance of three-year-old Mikaeel Kular.
"The man was questioned and released on police bail while the Crown Prosecution Service considers the case."
Police Scotland confirmed a 19-year-old man had also been arrested with the assistance of Hampshire Police, over remarks "of a racist nature" allegedly posted to a social media site.
He has since been bailed while enquiries continue.
A force spokesman said, in both cases, the remarks made online were reported to police by concerned members of the public.
He added that officers from Police Scotland then liaised with colleagues from Hampshire Constabulary and Derbyshire Constabulary to identify the individuals responsible for the posts.
Chief Supt Mark Williams, Police Scotland Edinburgh divisional commander, said: "Any reports of racist, offensive or abusive comments made online will be thoroughly investigated.
"If you commit an offence, we will investigate it and you will be prosecuted."
A Hampshire police spokesman said that the man arrested in the county was 19 years old and was detained in connection with a comment allegedly posted on Facebook:
He said: "We received a call on Saturday night reporting a racially offensive comment posted on Facebook that could relate to the death of Mikaeel Kular in Scotland.
"We are investigating the message to assess whether any criminal offences have been committed.
"A 19-year-old man was arrested in Hampshire on Saturday night on suspicion of publishing written material to stir up racial hatred.
"He was questioned at a police station in Hampshire and later released on police bail until the last week in January 2014 pending further enquiries.
"Hampshire Constabulary is in contact with Police Scotland about this investigation."