Police said the man was known to the five-year-old's family. The 46-year-old from the Machynlleth area was caught on foot but his empty car was quickly recovered.
Forensic officers were already examining the vehicle tonight and described it as "a vehicle he had use of which is of interest to the inquiry."
Police are also looking at whether April's abduction is linked to another attempted abduction in the Aberystwyth area around a week ago.
It comes as police confirmed that they were also looking closely at the movements of known registered sex offenders living in the area.
It is also understood that the man being questioned is known to April's family but is not a relation.
That would explain why the five-year-old apparently got into a car with the person who abducted her of her own free will.
Officers said the man was arrested by a police patrol to the north of the mid Wales market town around 3.30pm.
The arrest came in the hour before April's parents Coral and Paul Jones were due to make a public appeal for help at a police press conference in Aberystwyth.
April was last seen by another child getting into the driver's side of a grey van in Machynlleth at about 7pm last night.
Police said it appeared that she had got into the van willingly, but they did not know if she was led to the vehicle or went there herself.
Speaking after a press conference this afternoon, Detective Superintendent Reg Bevan, from Dyfed Powys Police, said: "We were pursuing a number of inquiries, some of those were searching addresses. We were looking for this individual so we knew who we were looking for and as a result we have had a number of patrols in the area and we have come across this individual."
He added that the force's priority remains finding the little girl alive. "We are still pursuing all lines of inquiry with a view that April is still alive and we will continue to do so until we find her," Mr Bevan said.
"We will not focus all of our efforts just on this individual. He is significant clearly, that's why we've arrested him, but we will still be pursuing all other lines of inquiry."
A massive search has been mounted for April, with volunteers being co-ordinated into groups of 20 to scour sections of the surrounding area.
April's godmother Mair Raftree, 41, from Aberystwyth, who has helped with the efforts to find her, said: "She's a quiet girl, and an important member of her family. She would never go with anybody and ask for a lift. We just want her back safely."
Neighbours on the quiet former council estate of Bryn-y-Gog, from where April was taken, were in shock today.
They described parents Coral, 40, and Paul, 43, as "stunned and devastated".
Mathew Harris, 22, who lives on the estate, said: "The general feeling is one of disbelief, nobody can take it on board.
"I was born and grew up on this estate and remember what it was like as a kid. Everyone played with everyone else.
"It's the same today. Kids play out together and everyone looks after everyone else. Somebody said that this estate once won an award as the safest in Powys.
"I can believe that. Every new family that comes to live here says what a great place it is to bring up kids. They can run free outside, riding bikes or playing football, without any trouble.
"What happened last night is unbelievable. Paul and Coral are loving parents and everyone is devastated for them."
At the press conference, Mr Bevan said that April's parents were the first to be told after the arrest was made.
He described the investigation as "fast moving" and "fluid" and said all efforts were still focused on finding April.
He said the arrest was made just outside Machynlleth and that the man was being detained at Aberystwyth police station. He said police were "hopeful that this individual will assist us in locating April, who is still missing".
Superintendent Ian John, the local police commander, described the case as "unusual." He added: "We are hopeful that we can still find April safe and well."
He also confirmed that search efforts have been focusing on checking on the whereabouts of registered sex offenders known to be living in the area.
"In this type of investigation we will be looking back through those types of links."
April's godmother, Mair Raftree, said April's parents got married recently, and added: "She's got a sister called Jasmine, she's 16, and she's got a brother called Harley. I'm not sure if he's nine or 10.
"Jasmine was at the leisure centre last night until quite late. She was thanking everybody for their support. It's quite difficult for them as well, knowing that she's out there with somebody. They want her back as well."
Police also confirmed that they are looking at possible links between April's abduction and a suspected attempt last week.
That focuses on a report of a vehicle seen "acting suspiciously" in the Aberystwyth area, the apparent target being a young schoolboy.
Mr John said it was too early to say whether any such links existed and whether the earlier report was an attempted abduction.
He added that 40 police officers and another 40 trained mountain rescue personnel were now part of the investigation.
Police were tonight enforcing a restricted access down the A487 road heading north from Machynlleth close to the railway station.
They were advising drivers that the road was closed to all motorists apart from people who lived just beyond the closure.
It is understood that searches are now taking place along the stretch of road.
Several hundred people from across the region continue to help in the desperate search efforts for April.
Police are believed to have accepted a large contingent of 200 volunteers today. They were sorted into small search groups of 20.
They were then dispatched to areas not yet looked at as the search itself gradually spread across the area.
A further 200 were also waiting at Machynlleth leisure centre ready to start a fresh shift later in the evening.
Trained officers have also been seen scouring banks for signs of April along the River Dyfi.
