Oh, I'm not saying that they'd be the equivalent of X-Com forces at all. But they could still put up a fight, and some of them would have the training and backbone to actively fight back. I expect the best of them would be on the low end of the X-Com scale (the guys you send out as cannon fodder since they aren't worth more than that). So they might do ok against Sectoids or Floaters but would still be more likely to die or panic than anything else. Might not be worth actually determining that too much, other than random stats for the Police with a lower range than X-Com soldiers. Although SWAT teams would fare better, of course, since they have better training.
In general I'd like to see more forces involved in Terror missions. I can't imagine that any major city could be attacked by an Alien force without that country putting up a fight. And regular army troops could certainly handle fighting a lot of the enemies in X-Com without panicking at the first shot. They'd probably lose, since the Aliens have superior firepower, but they'd at least put up a fight. And I'd like to get that sense that there is a lot more fighting going on during a Terror mission than the amount we actually do as X-Com. But, of course, X-Com needs to be hitting the heart of the invasion to try and drive off the attack.
Civilians: Panic and run, preferably more intelligently than the ones in the original.
Regular Police: Prone to Panic but armed. Tend to take up defensive stands against the aliens but break if they take too many casualties.
SWAT/Military: Handle being under fire well, mostly put up defensive stands but if they start doing really well will chase down fleeing aliens.
I think the Police and Military should be there with a focus on containing the Aliens and getting Civilians evacuated from the attack area. That could actually provide a useful game mechanic, where you provide covering fire to help get Civilians out to the areas protected by Police or Military, and try to keep those non-X-Com forces alive so they can provide protection for the Civilians as they flee.