A police officer was in a coma yesterday after he and another officer, both off-duty and out of uniform, were attacked by a group of youths after a concert by the rock group The Doors at the Lycabettus Theater on Thursday night. Officers on duty also were attacked. The two off-duty officers had been attending the concert as part of an audience of about 4,000. Police said that shortly after the concert ended, about 50 youths had used rocks and sticks to attack uniformed officers on duty outside the theater. A senior officer was injured and his service revolver taken. Another three were slightly injured. When the youths realized that two members of the audience were off-duty officers, they attacked them, injuring both seriously. They took the service revolver of one of the men as well. Two of the injured officers on duty that night said the youths had been able to identify the off-duty officers because they searched the bags of people leaving the concert. The senior officer’s gun was found shortly afterward among shrubbery, along with the officers’ identity papers. No arrests were reported.