April Jones was last seen getting into a light coloured van which drove off in the mid Wales town of Machynlleth yesterday evening.
Dyfed-Powys Police said they are becoming "increasingly concerned" for the missing youngster's safety and have deployed a large number of officers and specialist teams to investigate the disappearance.
Hundreds of local residents in the small market town are said to have joined the hunt for April yesterday with many continuing their efforts overnight.
Detective Chief Superintendent Simon Powell said police were following a number of lines of inquiry.
He added: "Dyfed-Powys Police have launched an intensive search and criminal investigation into the abduction of a five-year-old girl, April Jones, in Machynlleth.
"April was last seen playing on a bicycle near her home address at around 7.30pm and was seen getting into what we believe was a light-coloured van which then drove off."
He continued: "We are obviously becoming increasing concerned for her safety and are asking for any one with information to please contact us on 101."
Up to 200 people congregated at Machynlleth Leisure Centre as news of the apparent abduction broke to help in the search for the missing girl, according to reports.
Police said April had been out in the town riding a bike with a friend before she vanished.
Local restaurant owner Razul Islam said a large majority of the town, which has a population of 2,000, was out in force searching for the five-year-old.
"There are hundreds of people out looking for her," Mr Islam. "It is such a small town and nearly everyone is out looking for her - I'm shocked that it has happened in a town like this."
Local petrol station owner Nerys Zjalic said she reopened her garage last night to help with the search.
Ms Zjalic, who runs Reads Garage, told ITV Wales: "My friend called to ask if we would open so people can get petrol to help in the search. Locals are out in full force searching for her."
She said that some people had returned home overnight, but were planning to reconvene their search at 6.30am.
"It just shows the community spirit of the town and how people get together in a crisis to do anything they can to help," she added.
Ms Zjalic, who knows the missing girl's family, said people had been coming from far and wide to help with the search.
"It's unbelievable - you just don't think that something like that would happen in a small town like Machynlleth.
"Nothing will be the same again - this is a small little market town and you think children are quite safe but this shows us it's not the case."
She continued: "People just can't believe such a thing has happened, the whole town is devastated and in shock."
Police released three pictures of brown-haired April and urged anyone with information about the youngster's disappearance to come forward.
Locals have also put up posters of April around the town as part of their search efforts.
Chris Dudley, who works at the Skinners Arms on Penrallt Street, said the pub was empty last night with many of the locals joining the search.
"All the locals are looking for her in this area - the cemeteries, the golf field, playing field, everywhere you can think of," Mr Dudley said.
"Police are still looking."
Mr Dudley said police officers were "everywhere" and they were also searching cars.
Inspector Kevin Davies from Dyfed-Powys Police said "every angle" of the investigation was being covered.
He said police dog teams were involved in the operation and that cars in and out of the town were being stopped and searched by officers.
A family liaison officer is also keeping the "devastated" relatives of missing April informed of any developments, Mr Davies added.
Machynlleth is situated in the Dyfi Valley and sits on the southern outskirts of Snowdonia National Park.
Search volunteer Anwen Morris said locals had searched an area with a 30 mile radius overnight using quad bikes and off-road vehicles. "The most important thing here is to get this little girl home," she said.
County Councillor Michael Williams also joined in the search efforts. "People were co-ordinated by the police and as well as searching the town they went out into forested areas of the valley and wooded areas and out into hamlets and byroads."
Speaking about April's relatives he added: "They are such a lovely, caring family and they are devastated. The community is shocked and stunned over what's happened."
A number of volunteers were congregating at the town's leisure centre this morning to continue the hunt, with maps of the area and pictures of April being handed to those offering their services.
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