Your PC or phone is infected with malware. If your anti-virus program can't help you (because not all malware are viruses), use UnHackMe (which deals specifically with other non-viral classes of malware), assuming you're on a PC.
It's a Chrome extension addon from a third party developer that violated Google policy by serving ads and pop ups through their addon.
Most likely to be a video download extension. I know because i traced the problem to that addon and uninstall it. Googled to find that others had the same problem.