By Natalie Pompilio

PHILADELPHIA (Reuters) - Lawyers representing six former Philadelphia police officers in a corruption trial said the U.S. government's case against them on charges they robbed and beat-up drug dealers was based on the unreliable testimony of everyday criminals.

The trial of the officers on charges including robbery, racketeering, civil rights violations and falsifying records began on Monday in U.S. District Court in Philadelphia.

Prosecutors have said the officers routinely beat up suspects they were investigating, stole cash and drugs from drug dealers, which in one instance they sold on the street, and falsified reports in a bid to cover up their behavior.

Another key prosecution witness is former officer Jeffrey Walker, who was caught planting evidence and stealing money during a 2013 FBI sting. Walker pleaded guilty to the charges against him and is expected to spend days on the stand testifying against his former colleagues.

"We are not dealing with people with high morals," defense attorney Jack McMahon said in his opening remarks, referring to Walker and other government witnesses. "They are the people who lurk in the dark while you and I go about our business."

McMahon also referred to Walker as "despicable, "a liar" and an "amoral creep."

The officers are: Perry Betts, 46; Thomas Liciardello, 38; Linwood Norman, 46; Brian Reynolds, 43; John Speiser 46; and Michael Spicer, 46. Speiser faces up to 40 years in prison while the other officers face life sentences.

A prosecutor on the case, Assistant U.S. Attorney Anthony Wzorek, said theft is theft, no matter if the victims are drug dealers. Last week, prosecutors dropped one witness and the charges against the officers related to his claims after finding inconsistencies with the man's account of events.

The start of the trial was another sore spot for the Philadelphia Police Department.

Philadelphia police need more training to defuse significant tension with the community, according to a U.S. Justice Department report released last week that found "serious deficiencies in the department's use of force policies and training."

(Editing by Michael Perry)