It isn't 'Sierra Yankee 99' as forensics said. I think the Metropolitan police have 3 Police Air Support Units and their call signs are India 99, India 98 and India 97 and all 3 are Eurocopter EC145s based at Lippitts Hill.
Don't double, or in this case, triple post. Go to this page and scroll down. There is a temporary solution there. You wouldn't have to reinstall the game anyway. You would only have to reinstall the mod. Try Hoppah's solution, that should work.
?ERROR: The Vision engine requires support for the ARB_multitexture extension! Please make sure you have the latest video drivers installed for your 3D card. Try updating your video card drivers.
I normally send a SWAT team and arrest the main guy (the one with the sign) and then send the SWAT Team back to base. After the main person is off the map the other protesters normally just walk away.