UFC's Irwin Rivera Arrested, Rep Claims Fighter Battling Mental Illness

1:54 PM PT -- Rivera's manager, Ali Abdelaziz, tells TMZ Sports Rivera is battling a mental illness and says the alleged attempted murder "was not in [Rivera's] hands."

"His sisters are in stable condition and I am praying for his family," Abdelaziz says. "We are still trying to put the pieces together."

Ali added, "I will never give up on Irwin and his family."

7:03 AM PT -- 1/8 -- TMZ Sports has obtained the police report stemming from Rivera's arrest ... and the allegations against the 31-year-old fighter are deeply disturbing.

In the docs, cops say they responded to a call in the Boynton Beach, Fla. area at around 3 a.m. Thursday regarding a stabbing -- and found a 22-year-old woman covered in blood in the roadway.

The woman, according to cops, had been stabbed in her back, head and arm ... and told the officers "her brother had stabbed her and her sister was still in the house."

Cops say they then found a 33-year-old woman inside the residence also suffering from stab wounds to her back, face, arms and hands.

Officers say the woman told them she had come to stay with Rivera in his spare bedroom ... and awoke early Thursday morning to the UFC fighter stabbing her repeatedly.

Cops say the woman told them she was eventually able to fight off Rivera and contact authorities.

In the docs, officers say they canvassed the area trying to find Rivera ... and when they eventually detained him, they say he "spontaneously uttered, I KILLED MY SISTERS."

Cops say Rivera was covered in bloody clothing.

In the docs, officers say during questioning, Rivera "admitted to stabbing both of his sisters."

Cops added that Irwin said "he entered his sisters' bedroom while they were sleeping and began stabbing them with a brass knuckle knife."

Officers in the report say Irwin told them "he did so because it was his purpose, which he was told to by a higher power."

Irwin is due to face a judge in court later Friday.