Heck there is a German Cop show that started in 1978 (making it 32) and that as far as I know is still on the air, dwarfing both The Bill & L&O in the cop show genre.
Its also had more than a few spin-offs (including one that crossed over with the before mentioned The Bill)
Seems to be some confusion, I think its true, but its not 100% yet, if true this is a massive screw up by NBC
There is also Tatort
with currently 770 episodes since 1970.
But it is made by a variety of regional networks, so it may not count as a series.