Authorities in Niger have arrested 17 people, including the wives of senior politicians, on suspicion of involvement in a baby-trafficking network that "bought and sold babies like bread".

The accused allegedly paid for newborns from neighbouring Nigeria, where human traffickers run notorious "baby factories" in which women and girls are often forced to bear children for sale.

The wife of Niger's agriculture minister was among those apprehended, as was a bank director and a policeman. The lawyer for another detainee, one of the wives of Hama Amadou, the speaker of parliament and a key opposition member, called the arrests a politically motivated "witch hunt".

Child trafficking has long been an issue across west Africa, and the high-profile arrests also implicated syndicates in Burkina Faso and Benin.

Investigators believe each case followed an identical trajectory: unable to have children, the moneyed couples, travelled first to Nigeria to "buy" the babies for thousands of pounds, before heading westwards to either Benin or Burkina Faso to get false birth certificates.

Ousmane Toudou, a justice ministry official, said: "A judge has indicted them on charges of lying about giving birth, use of false documents, and forgery."

Toudou said they would probably be held for several months ahead of trials, and dismissed allegations of political motivations. "One man had three wives, one of whom is in her 50s, the other in her 60s. In the space of two months, all three wives had had twins," he said. "It's a biological aberration, clearly."

The incidents came to light after a local journalist, Moussa Akfar, was approached nine months ago by a man who became suspicious after his neighbour claimed to have had a child. "He didn't understand how the wife never showed signs of pregnancy, and suddenly one day the couple had a child.

"We approached it very delicately and what we realised was this wasn't just one or two isolated cases – it's on a massive scale across several countries," Akfar said.

Niger's authorities initially showed little reaction, Akfar said. Then, after 92 migrants died when traffickers abandoned them in the Sahara desert, he began publishing further stories.

"I asked why there hadn't been the same outcry as when the migrants died and sanctions were imposed. Selling babies is another form of trafficking. If people in positions of power bought and sold babies like bread, then we need to start asking serious questions about where we are headed as a society," Akfar said.

Statistics are hard to come by, but campaigners say the sale of newborns is widespread and well-efficiently run by both individuals and criminal organisations largely concentrated in Nigeria.

Typically, "factories" hold dozens of women in squalid conditions for several weeks before selling their infants for 750,000 naira (£2,700) for a girl, or twice that for boys.

Most of the trade is driven by local demand in a region where being childless has a social stigma and fertility treatment is limited. Pregnant women sometimes sell their babies willingly if they have been raped, have unwanted pregnancies, or simply see it as a way to make money when most live on less than $2 a day.

But babies are also sold abroad, prompting Denmark to suspended adoptions from Nigeria in April this year.

After falling pregnant while still at school, 17-year-old Elizabeth (not her real name) ran away fearing ostracism from her community in Abia state. A doctor who lured her to a dusty local clinic promised to perform an abortion but instead drugged her until she gave birth a week later, when he handed her a 20,000 naira (£80) "gift".

"When I came home, my mother begged me where was the child was until I took her to the hospital," she said. "We went to the hospital and the doctor brought out a different baby but told us we should come back a week later because the child was premature."

The doctor was arrested when another woman – the actual mother of a baby – walked into the room and demanded her child back.

Most cases are referred by churchgoers, said Arinze Orakwue, of Nigeria's National Agency for the Prohibition of Traffic in Persons. "You have cases where women go to church for baptisms and they weren't pregnant two months ago. That's how people know, this is a buy-buy [purchased] baby," he said.

Traffickers try to sell babies off as quickly as possible to reduce feeding costs and the risk of discovery, making it even more difficult to trace them, Orakwue said.

In the high-profile Niger case, investigators were able to follow a paper trail that led them to a clinic in Benin's bustling port city of Cotonou. "Their modus operandi here was to scan false adoption papers and then get a clerk to sign them. It's a well-established routine," an official from Benin's child protection department said.

Niger ranks 28th highest in the latest global index of slavery; Benin and Nigeria are 7th and 48th respectively.