Kid Cudi Testifies Diddy Broke Into Home After Finding Out About Cassie Fling

Kid Cudi says Diddy broke into his Los Angeles home after finding out he was dating Cassie ... and he says Diddy screwed with his pooch.

The rapper took the stand Thursday in Diddy's federal criminal trial in NYC and told the jury about some of the drama that unfolded between him, Diddy and Cassie back in 2011 ... when she was on a break from Diddy and they started dating.

Kid Cudi testified there was a break-in at his home in December 2011 after Cassie called him early in the morning and told him Diddy found out about them. He says cops were called, too.

Cudi says Cassie was stressed, worried, nervous and scared when she called around 5:30 or 6 AM to tell him Diddy knew they were seeing each other ... and he says she told him she'd given his address to Diddy and didn't know what Diddy would do.

Kid Cudi says Cassie told him to come pick her up and they went to the Sunset Marquis Hotel in West Hollywood.

Cudi says while he was with Cassie, they got a call from Diddy employee Capricorn Clark, who told them Diddy was inside Kid Cudi's house. Cudi said Clark told them she was waiting in a car outside the house while Diddy was inside.

Kid Cudi testified he called Diddy and planned to confront Diddy at his home ... telling Diddy, "Motherf***** you in my house?" He says Diddy calmly told him, "What's up, I just want to talk to you."

Cudi says he told Diddy he was on his way home ... but when he got there, no one was there ... though he says he soon discovered his security cameras were not working and that they had been moved without his knowledge.

When Kid Cudi went inside, he testified some gifts he received, including one from Chanel, had been opened ... and his dog, who usually roamed the house freely, was locked up in a bathroom. He also said his dog became jittery and on edge in the wake of the break-in.

Cudi says he called Diddy again and wanted to go looking for him to fight him ... but he says he ultimately decided to call the cops.

Kid Cudi says he spent Christmas 2011 with Cassie and her family in Connecticut and that Diddy called him a few times. Cudi says he told Diddy, "You broke into my house and messed with my dog, I don't want to talk to you."