Andrew and Tristan Tate are gearing up for a major legal battle ... 'cause their lawyer says they're "100 percent" fighting extradition back to the United Kingdom.

Lawyer Joseph McBride tells TMZ ... fighting extradition is a high bar, but he'll argue there are special circumstances why the brothers shouldn't be sent back. He says they're not a flight risk, pointing to what he calls their total compliance with the courts over the past three years. According to McBride, they haven't missed a single court date in either their Romanian human trafficking case or Palm Beach defamation case.

McBride says the brothers were "startled" when U.S. Marshals in plain clothes wearing vests and badges approached them during Saturday's arrest and ordered them to put their hands behind their backs.

He says it "doesn't feel good" for the brothers, who were jet-setting around and hanging out with supermodels on yachts, to suddenly find themselves in what he describes as a "smelly jail." McBride says they spent Sunday in solitary confinement, calling it "cruel and unusual punishment," though he says U.S. Marshals told him they'll be moved into general population.

The Tate brothers were arrested in Miami after U.S. Marshals executed a U.K. extradition warrant seeking to return them across the pond, where prosecutors have accused them of rape, human trafficking, and other alleged sex crimes. The brothers have repeatedly denied all allegations.

McBride calls the U.K. case politically motivated, saying the brothers are American citizens with a First Amendment right to "say whatever they want and not go to jail." He claims authorities from The United Kingdom are "misusing the justice system, politically kidnapping and putting them in an English Gulag."

McBride says the brothers knew they weren't getting out at Monday's hearing and are now "digging in and getting ready for a meaningful fight."