April Jones was playing with friends near her home in Machynlleth but was seen getting into a grey or light coloured van at 19:00 BST on Monday.
Around 200 people helped in a search of streets, farmland and woodland through the night.
April was last seen wearing a purple knee-length coat with a fur hood, white school polo top and black trousers.
The Ysgol Gynradd Machynlleth pupil had been playing on her bicycle with friends near her home on the Bryn-y-Gog estate.
Dyfed-Powys Police said all the witnesses to her suspected abduction were children.
Det Supt Reg Bevan said getting information from the children would take time.
"It's a delicate and difficult process interviewing young children who were the witnesses to this," he said.
"That's continuing - it commenced last night and it will continue this morning and hopefully I will have more information for you later."
A hotline number has been released for people to call with information - 0300 2000 333.
Det Supt Bevan added: "We're liaising with our colleagues, neighbouring forces, [getting] specialist advice.
By Cemlyn DaviesBBC Wales
I've already heard shouts and cries of 'April' as the search resumed this morning.
I'm standing near the spot where the five-year-old was last seen around half past seven on Monday evening.
The community has really pulled together and that was clear last night.
A few hundred people gathered at the town's leisure centre.
I was there to watch them come together. They divided themselves into groups to go out into the town to keep searching for April.
This morning, I've spoken to the local councillors who have said they will do everything possible to help April Jones's family at this very, very difficult time.
They've described the family as a wonderful family who play their part in the community, and they'll do everything they can to help find April safe and sound.
"We've got specially trained family liaison officers supporting April's family."
He also praised the "tremendous response" from the community.
Local people gathered at Machynlleth Leisure Centre on Monday evening to help look for the girl, as news of her disappearance spread on social network sites.
Posters of April were also circulated and put up around the town.
Police also set up road blocks to check vehicles, while the street where April disappeared has been cordoned off.
Det Ch Supt Simon Powell of Dyfed-Powys Police said officers were "becoming increasingly concerned for her safety".
He said the force had launched an "intensive search and criminal investigation" into the abduction.
"April was last seen playing on a bicycle near her home address and was seen getting into what we believe was a light-coloured van which then drove off," he added.
Insp Kevin Davies said the fire service and police dog teams had been mobilised, and vehicle check-points set up.
"There is mention of a grey van, we're looking at that as one of the angles of inquiry," he said. "Nothing is being overlooked. It's an angle, it's an inquiry that we're pursuing along with other angles.
"We're searching the area.  Everything we can possibly do is being done.
"The family are obviously distressed. We're trying to keep them informed, we've got an officer allocated to them just to keep them informed as to what's happening. Any information, we'll share with them."
Shadow Welsh Secretary, Owen Smith MP, told the Labour conference in Manchester that the thoughts and prayers of all delegates are with the missing girl and her family.
"We all pray she's back in her community and with her family as soon as humanly possible," he added.
Meanwhile, Powys County Council said it was providing support for pupils and staff at April's school. Both the primary school and adjacent secondary school are open with lessons continuing as usual.
The council said refuse and recycling collections in the town had been suspended for the day, with the workers involved volunteering to join in the police search.
Machynlleth, an historic market town in countryside in the Dyfi Valley, has a population of around 2,100.
Search volunteer Anwen Morris said locals had searched an area with a 30-mile radius overnight using quad bikes and off-road vehicles.
Local people described the family as "very well liked" and said they had helped create a community garden on the town's Bryn-y-Gog estate, which has more than 100 homes.
Alun Hughes, a friend of the family, said: "We were out last night and at the moment we are waiting for the police to organise specialist searches of the town."
Brian Vaughn, whose daughter goes to school with April, described her as a "lovely little girl". He said: "Everybody just wants her back, really.
"Whatever we've got to do, we'll do it. We're just hoping for the best, that she's closer rather than further away."
Mayor of Machenlleth Gareth Jones, who was among those at the leisure centre, said people wanted to help the search in any way they could.
"We're all shocked, and it's the uncertainty of it all as well of course which is really worrying," he said.
"Our thoughts go out to the family, the close family and friends of April.
"We're all here to help as you can see by the amount of people we've got here this evening.
"Anything we can do, we will do it."
Machynlleth town councillor Michael Williams said people were "stunned".
Machynlleth is a quiet Powys market town.
Tourism is the town's major employment sector, with the countryside, along with its proximity to the coast, attracting people from across the UK.
Caravan and camping sites are dotted across the rural landscape and its rural setting and relatively slow pace of life has led to many visitors choosing to settle there in search of an alternative lifestyle.
The town, with its weekly market, remains one of the Welsh-speaking heartlands.
Crime is low with an average of 15 crimes a month reported in Machynlleth so far this year,  mostly for anti-social behaviour.
"I've lived here all my life and I've never heard of anything like this before," he said.
"The family must be devastated, and I hope to goodness the little girl is found without any huge delay.
Neighbour Rhydian Mason said: "Most of us I think have found out through social networking sites online like Facebook - that's where I first heard of it - and we all want to see what we can do to help.
"It's completely heartbreaking. I'm trying to put myself in the shoes of April's parents and it's an awful feeling in the pit of the stomach. We're all out here hopefully to see if we can get a positive outcome to this story, and we'll do our best even if we're out all night."
Dyfed-Powys Police have asked anyone with information to contact a dedicated hotline on 0300 2000 333.
Police are treating the disappearance of a five-year-old girl in Powys as a suspected abduction.