A major operation is under way in Edinburgh to locate Mikaeel Kular after his mother found his bed empty this morning.
A police helicopter, sniffer dogs, coastguard and lifeboat teams have been tasked in the search for the toddler who was last seen at his family's flat in Ferry Gait Crescent at around 9pm.
Police say there have been no arrests but several people are helping them with their inquiries.
His jacket and shoes are missing and police are looking into whether he might have left of his own accord. Police said there is currently no evidence of criminal behaviour.
A dedicated telephone number has been set up for anyone who has seen the child or has information about his whereabouts.
Neighbours on the modern housing estate have joined the search for Mikaeel, a twin with three other siblings.
One resident, who did not want to give her name, said: "I got woken up at the back of 7am by the police, asking if I'd seen the wee one.
"I got the fright of my life because I've got a child. It's just absolutely horrific.
"I didn't see anything out of the ordinary. The first thing I did was actually search the stair."
The neighbour said a group of about 10 mothers have been out in their cars searching the area.
Flora Stevenson Primary, where Mikaeel attends afternoon nursery, says it has been contacted by several parents following the disappearance.
In a letter to parents, head teacher Irene Brennan said: "This release of this news has led to questions and concerns from some of our children and a number of parents have also contacted the school. I understand why you may be concerned but it is important to recognise that it appears that Mikaeel went missing from his home.
"I would like to reassure you that the children are safe in our school and we are reassuring them about their own personal safety. As well as supporting our children, our staff are also supporting the Police with their enquiries. We are keeping in touch with Mikaeel’s family and our thoughts are with them at this difficult time. I am sure you will join us in hoping that he will be found safely in the near future and returned to his family.
"Police Scotland Superintendent Liz McAinsh said the force is using all the resources at its disposal in the hunt.
"Asked whether there were any custody issues with the family, she said: ''Obviously we are following all lines of inquiry, there are no concerns that we have at the moment.
"We are following all lines of inquiry with all partner agencies and with the family and the family are being very helpful at the moment.
''As you can imagine, Mikaeel's mother is distraught. She just wants our help and the public's help in finding her little boy.''
The police search is focusing on the Pennywell and Muirhouse areas of the city. Coastguard and lifeboat teams have meanwhile been scouring the shoreline in the Cramond and Drylaw areas.
Mikaeel is described as 3ft, of Asian appearance and possibly wearing a beige hooded jacket, blue jogging bottoms, brown Clarks shoes and nightwear.
The youngster attends a local nursery and Ms McAinsh said: ''They say Mikaeel is a friendly, playful wee boy and they are obviously concerned for his safety as well and will do everything they can to help us in finding Mikaeel.''
The dedicated number for anyone with information on the child's disappearance is 0300 200 0200.
The telephone line has been established as part of a Child Rescue Alert, supported by all police forces across the UK.
People are being asked to urgently check all property including gardens, outbuildings, garages, sheds and vehicles for the youngster as darkness falls.
Temperatures in Edinburgh dropped to 2.3C (36F) last night.
Robert Pearson, chairman of a local residents' group, said he sent out a plea for help earlier and around 50 people responded.
He said: "We split up into groups of seven to search the area. My group of seven are currently going through the Silverhouse Golf Course at the moment but we are fearful and wary of the fact that we might not have much hours left of light as it begins to get darker.
"We are liaising with Police Scotland and will continue to do so until they say it is unsafe to carry on searching. At the moment they have roughly 70 officers searching the area, but the mood amongst the volunteers is that it is feeling quite desperate just now."
Neighbour Linda Slattie, 52, said: "We were out this morning looking, down to Muirhouse Park, the community centre, looking in bushes and lock ups and things like that.
"I've been very shocked. It could be any one of our kids."
Police released a more detailed description of Mikaeel, adding that the youngster has a faded scar on the bridge of his nose and a sore on the left hand side of his mouth.
He may be wearing a grey pyjama top with an embroidered turquoise dinosaur on the chest and black gloves with multi-coloured fingers.
In the House of Commons, Edinburgh MPs expressed their hopes Mikaeel would be safely returned to his family as soon as possibly.
Liberal Democrat Mike Crockart (Edinburgh West) said: ''My thoughts particularly are with the family in my constituency where a three-year-old went missing from his home overnight last night and I know we all fervently hope for Mikaeel Kular's safe return as soon as is possible.''
Labour's Mark Lazarowicz (Edinburgh North and Leith) told MPs: ''I hope he is soon found safely and brought back to his home.''
Police earlier detained a man in the city's Waterfront Gait but said it was unconnected with the boy's disappearance.
Ms McAinsh said: "I would stress that our search focus remains in Edinburgh, in the Drylaw area, specifically in Pennywell and Muirhouse and Silverknowes.
"I would ask anyone who has information of who has seen Mikaeel to contact us on the dedicated number.
"Anyone in the locality, please continue searching in your own properties. If your neighbours are away, think about searching their gardens, their sheds, their wheelie bins - anywhere a three-year-old boy might seek shelter.
"It's getting dark, it's getting colder, please help us."
The superintendent said they had spoken to all "key" members of the boy's family who are assisting police.
Mikaeel, who has two sisters and two brothers, was discovered missing from his bed by his mother at around 7.15am today when the family woke up.
Ms McAinsh said: "There have been no arrests. There have been no formal detentions, but several people are assisting us with our inquiries.
"Those inquiries are continuing."