I was pretty sure this was wrong, but wanted to google it just in case.
One of the first hits was this website:
http://www.wisegeek.com/does-an-off-duty-police-officer-have-authority.htm
And, right there in the second paragraph, was:
You should plagiarize from better sources
Title 42 of PA General Assembly Consolidated Statutes
§ 8952. Primary municipal police jurisdiction.
Any duly employed municipal police officer shall have the power and authority to enforce the laws of this Commonwealth or otherwise perform the functions of that office anywhere within his primary jurisdiction as to:
(1) Any offense which the officer views or otherwise has probable cause to believe was committed within his jurisdiction.
(2) Any other event that occurs within his primary jurisdiction and which reasonably requires action on the part of the police in order to preserve, protect or defend persons or property or to otherwise maintain the peace and dignity of this Commonwealth.
So long as an officer is "duly employed", they have the power to arrest. The on duty/off duty thing is not a legal consideration on the power to arrest. There are many practical reasons why an off duty police officer would not make an arrest, but from a purely legal standpoint, they do have the power to do so. They certainly don't have to place someone under "citizen's arrest" until an on duty officer shows. up
Now, if the off duty officer was outside of their municipal jurisdiction, then according to 8953(a)(6), the officer could only make an arrest if the officer "views an offense which is a felony, or has probable cause to believe that an offense which is a felony has been committed, and makes a reasonable effort to identify himself as a police officer."