Prosecutors are vehemently opposed to letting Diddy out on bond ... they say his convictions for the Mann Act make him a violent criminal.

The feds just fired off a letter to Judge Arun Subramanian outlining the reasons why they say he should remain in custody pending sentencing for his conviction on two counts of transportation of prostitutes across state lines.

In the docs, obtained by TMZ, prosecutors say Diddy is trying to use the Bail Reform Act to get released on bail pending sentencing ... but they say detention is mandatory for Diddy based on the crimes for which he was found guilty.

Prosecutors say the Bail Reform Act explicitly says a violation of the Mann Act is considered a crime of violence ... and detention is mandatory in such cases.

The feds say their preliminary calculations peg the sentencing range to around 4 to 5 years, and they claim he's a flight risk because of these "significant potential penalties."

Prosecutors also say the overwhelming evidence established at trial -- which the defendant did not dispute -- shows Diddy engaged in a years-long pattern of abuse and violence ... and they say Diddy is a danger to others and is either unable or unwilling to follow the law.

Meanwhile, the judge also got letters from a couple folks close to Cassie ... arguing for Diddy to remain locked up.

Deonte Nash, a celebrity stylist who was called as a witness for the prosecution, expressed "grave concern" for Diddy's potential release ... claiming Diddy has a "well-documented history of violent, coercive, and retaliatory behavior. Over the years, he has repeatedly escaped meaningful accountability, and each time this has only reinforced his sense of impunity."

Nash also says of Diddy, "If he is released now, I have no doubt he will see it as yet another license to continue intimidating, threatening, and harming people who challenge or expose him."

Cassie's lawyer, Douglas Wigdor, says Cassie believes Diddy "is likely to pose a danger to the victims who testified in this case, including herself, as well as to the community."

Meanwhile, Diddy's defense is arguing he should be released on $1 million bond ... they say he's not a flight risk, isn't a danger to anyone, and won't call up and escorts if released.

The judge will consider arguments from the defense and prosecution before making a final decision, which he says will come down around 5 PM ET.