A 46-year-old man has been arrested in connection with the abduction of five-year-old April Jones.
Police said the search is continuing for the little girl who was last seen playing on her bicycle with friends near her home in the Bryn Y Gog estate in Machynlleth, mid Wales, at around 7pm last night.
She was last spotted by another child getting into a grey or light-coloured van - similar to a Ford Transit Connect or Land Rover-type vehicle.
Detective Superintendent Reg Bevan, of Dyfed-Powys Police, said officers would give an update on the hunt at 9.30pm tonight.
He said the suspect was arrested on suspicion of abduction at around 4pm, and they had been "actively looking for him".
The suspect's name is Mark Bridger, according to Sky News sources.
Mr Bevan said he is a local man and drives a vehicle similar to the one in the police appeal. He was walking by the side of the road, a couple of miles outside the town, when he was seized.
"We were keen to trace him," Mr Bevan added.
Police are also looking at whether her disappearance is linked to another attempted abduction in the Aberystwyth area a week ago.
Officers are examining a van found near a bridge on the A487, two or three miles north of the town. Aerial pictures showed a white Ford Transit Connect van surrounded by police cars.
Police are searching the surrounding area, including the banks of the River Dovey.
April's parents Paul, 43, and Coral, 40, were due to read a statement at a news conference at 4.30pm, but instead police gave news of the man's arrest.
April also has a brother Harley, who is nine or ten, and a 16-year-old sister Jasmine, who was helping with the search overnight.
Her godmother Mair Raftree, 41, from Aberystwyth, wept as she told Sky News about the family's distress. She said they are hoping "April will be returned safe and well".
She added: "She's lovely, she's got a nice little character. We just want her back."
Superintendent Ian John told Sky News: "It's a very, very difficult time for the family and we are trying to give them all the support we can."
He said the suspect is being questioned at Aberystwyth police station. "It's a significant arrest. We want to focus on finding April who's still outstanding and we continue to search for her," he added.
He appealed for the public to continue searching for April, who was wearing a purple knee-length coat with grey fur around the hood, a white polo top, and black trousers, when she was abducted.
A huge appeal has been launched on Twitter, with many celebrities tweeting and re-tweeting the message. Her poster is also being circulated on Facebook.
During an earlier news conference, Mr Bevan said April got into the driver's side of the van. It means she could have got in with the driver or the vehicle was a left-hand drive.
"The indications are that she got into the vehicle willingly ... It's very difficult to give an exact account of what happened. There's nothing to suggest at this stage that there was a struggle when she got into the car," he said.
He also emphasised the difficulties of getting accurate information from the young children who were playing with April when she disappeared.
A massive search has been under way since April went missing. Overnight, police set up roadblocks around the town and checked cars. Two helicopters have been scouring the area along with dogs.
Hundreds of residents have also joined in the search and are co-ordinating their efforts at the Machynlleth Leisure Centre.
They were divided into groups of 20 by the police and sent out into the area, including the mountains with the help of local mountaineering experts. The small town, which has a population of 2,000, sits on the southern outskirts of Snowdonia National Park.
Volunteer Anwen Morris appealed for more people to come forward and help. Many have taken time off work to look for April and have been out through the night on quad bikes and off-road vehicles.
Ms Morris told Sky News: "We need bodies down here. We need people in cars, we need people on foot to search the town. We need people up on the mountain. We need anybody that is in the area that can help."
Powys County Council said refuse and recycling collections had been suspended in the town on Tuesday so council staff could help with the search.
Chris Dudley, who works at the Skinners Arms pub on Penrallt Street, said all the locals were out looking for her on Monday night, including in "the cemeteries, the golf field, playing field, everywhere you can think of".
:: Police have asked anyone with information to call them on 0300 200 0333.
Live Updates: Search For Missing April Jones