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Toto's former frontman Bobby Kimball had an intense showdown with cops at the airport after allegedly touching a child's butt with his cane while in line at a TSA checkpoint.

The incident went down in June and according to cops ... officers wanted to question Kimball after someone accused him of inappropriately contacting a 4-year-old girl.

Cops say the girl pointed out Kimball to her mother, and a witness told cops he saw the whole thing. Officers then tracked down Kimball in a bathroom. Cops says that's when things got physical.

According to the police report ... an officer tried to detain Kimball for questioning, and he spun around and said, "I am going to knock you the f*** out!!!" Police say Kimball put up his fists, but the officer was able to pin him to a wall and call for backup.

In the report, police say Kimball continued resisting arrest and it took two officers to finally get him on the ground and slap on a pair of handcuffs.

During the scuffle, police say one of the officers suffered a cut on his left forearm and Kimball complained of back and shoulder pain after he was handcuffed ... so, he was taken to a hospital.