"I was screaming and I was in pain ... I told them there was no need for punching me or hurting me and please stop,” Schmitz said at the trial on Tuesday, describing the raid that took place in January.
New Zealand authorities raided Dotcom's rented estate outside Auckland following a request from the US Federal Bureau of Investigation (FBI), seizing a number of computers, hard drives, art work and luxury cars.
Dotcom testified that the police forced him to the floor and one officer stood on his hand while a second kneeled on his ribs. He also said he felt kicks and a punch in the face, Bild newspaper reported.
Asked why the police photos did not show any evidence of this, he replied that they had been taken immediately afterwards, and that the bruises had not shown up until the next day. He was arrested at the raid, and released on bail a month later.
The raid has since been ruled illegal, and it remains unclear whether the confiscated computers can be admitted as evidence in the trial.
US authorities say Dotcom's internet platform Megaupload netted $175 million since 2005 by distributing music, films and other copyrighted content without authorisation. But his defence team say that he had simply offered an online storage facility.
Should Dotcom - a German who legally changed his name from Schmitz - be extradited to the US, he could face up to 20 years in jail.