To enter safe mode:
Press and hold the power button until the 'Phone options' menu pop up.
This gives you three main options - power off, restart or turn airplane mode on.
To enter safe mode you have to press and hold 'power off' for about 3 seconds. It will then ask you whether you want to boot into safe mode or not. (Tested on D802).
On some device models, you have to tap and hold 'reboot device' in order to get that option.
It works flawlessly and in order to turn it off, you just reboot your device. During safe mode, the phone can be used as usual, with the exception of 3rd party apps.
Hope this helps!
Sent from my LG-D802 using XDA Premium 4 mobile app