Kate Upton Claims Paul Marciano Grabbed Her Breasts, Kissed Her Durin -

Kate Upton is finally detailing her allegations against Guess co-founder Paul Marciano ... claiming he was all over her during a meeting, forcibly kissing her and groping her.

Kate alleges during the 2010 meeting, Marciano "grabbed my breasts and started feeling them -- playing with them actually. After I pushed him away, he said, 'I'm making sure they're real.'"

The supermodel also told TIME ... although a photographer was with them, Marciano went on to grab her thighs, pull her closer by her shoulders and kiss her face and neck. She says he tried to walk her up to her hotel room, but the photog stepped in, in order to protect her from Marciano.

She says she was fired from the shoot the next day -- and eventually the photographer who helped her was also fired. Kate feels it was retaliation.

Kate says the last straw came in May 2011, when she claims Marciano was rude and degrading toward her -- and she was told to leave set after Marciano said, "Get that fat pig off my set."

Marciano has claimed Upton was hungover and unprofessional on that final shoot, but she denies that and accuses him of "spreading rumors."

As we reported ... Marciano got out ahead of Kate's allegations when she initially hinted about misconduct on social media, claiming he was "sexually and emotionally" harassing women.

ONLY THE BEGINNING ...

Marciano insisted to us ... he had never touched Kate or acted inappropriately toward her.

Upton also told TIME ... "I'm sick of being expected to laugh off these aggressive advances and accept the power imbalances that exist. I'm sick of being expected to endure all of this while being polite and professional through it all."