The footage, which has been posted online is said to have been taken in Derry-Londonderry on St Patrick's Day on Monday.
The uniformed individuals then get into an unmarked car and drive off while the woman remains lying in the lay-by - only a few feet from the pavement.
The Police Service of Northern Ireland (PSNI) has said it will fully support the probe by ombudsman Dr Michael Maguire into the alleged incident. A spokesman for Dr Maguire said:
The Police Ombudsman has this morning viewed video footage which appears to show what happens after two police officers discover a young woman lying on a road in Derry/Londonderry. Dr Maguire has now initiated an independent investigation into the police handling of the incident.
Foyle Assembly member Mark H Durkan said the footage will have shaken public confidence in the PSNI.
"Not only are these actions dangerous and irresponsible they are completely inhumane," the SDLP representative said of the alleged incident.
A spokeswoman for the PSNI's oversight body, the NI Policing Board, said:
Whilst the circumstances surrounding this incident are not yet clear, video footage of the police response raises questions of the most serious nature. The Board and the public rightly expect the highest professional standards of all of our police officers and anything that falls short of that is simply unacceptable.