* Autopsy images showed body covered in dark bruises
* Border officers plead not guilty of homicide charges
* Critics say case exposes abusive culture in border agency
* Border agency’s workers union denies accusation (Adds summary, quotes from first day of trial)
LISBON, Feb 2 (Reuters) - Three Portuguese police officers accused of killing a detained Ukrainian man in a Lisbon airport lock-up pleaded not guilty as their murder trial began on Tuesday, a case rights advocates say has exposed a record of abuse within the immigration service.
Lawyers representing the accused argued the media had made scapegoats out of their clients, and that others working in the detention centre that night should also face charges.