The National Crime Agency (NCA) is examining evidence provided by the Sun on Sunday, who filmed former Nigeria and Portsmouth defender Sam Sodje claiming he received £70,000 for getting himself sent off in a Championship fixture last season.
Sodje was shown a red card after repeatedly punched Oldham's Jose Baxter in the groin, and he admitted receiving a substantial reward to the undercover reporter.
He also told the reporter that he knew current players who would be willing to get booked in return for cash rewards, and hinted he could rig matches at next summer's World Cup finals.
The 34-year-old, who was released by Pompey at the end of the season and remains a free agent, is believed to be one of the six men arrested along with Oldham midfielder Christiano Montano.
The Colombian winger was also filmed claiming he agreed to get himself booked in return for cash, although he failed to incur a yellow card in the fixture against Wolves in October.
An NCA statement read: "We can confirm that the Sun on Sunday has passed material from its own investigation to the National Crime Agency.
"An active NCA investigation is now underway and we are working closely with the Football Association and the Gambling Commission. Six people are in custody and are being questioned by NCA officers. We cannot comment further at this stage."
Portsmouth have said they are "shocked and saddened" by the allegations, while Oldham have also vowed to co-operate fully with the police.
Pompey spokesman Colin Farmery said: "If these serious allegations are true then we are extremely shocked and saddened by them, as match-fixing of any type goes to the heart of the integrity of the game.
"The player in question no longer plays for the club and we have not been contacted by the authorities, but of course we would co-operate fully with any inquiry."
Oldham said in a statement: "Oldham Athletic Football Club has been made aware of the incident and allegation surrounding one of its players, Cristian Montano. The club will commence an immediate internal investigation to establish all the relevant facts of the case.
"The club is co-operating with other agencies in this matter and cannot comment on specific facts at this stage. The club will not be issuing any further statements at this time."
Football League chief executive Shaun Harvey added: "We treat any allegations of criminal activity in our competitions with the utmost seriousness.
"Given that there is an ongoing police investigation into this matter, we cannot comment further at this time.
"We would encourage anyone with any evidence to report it to the police. We will be giving our full assistance to the Police during their investigation."
The NCA is already carrying out a separate investigation into an alleged international illegal betting syndicate.
Non-league Michael Boateng and Hakeem Adelakun, both 22, were charged this week with conspiracy to defraud contrary to common law.
The pair, who played for Conference South club Whitehawk FC in Brighton, were sacked by their club and will appear in court on Wednesday.
Two other men, Chann Sankaran, 33, from Hastings, East Sussex, and Krishna Sanjey Ganeshan, 43, from Singapore, were charged last month with plotting to defraud bookmakers and will also appear in court this week.