My playes got into some trouble last session and ended up killing a few city guards. Typical low-level character stuff. Now, they've got to get oth of the cty before they can get arrested. I don't know if they'll go for stealth or disguise, but they're definitely gonna run.

So how do I do this? If they get found out, I can easily run a chase. But before that happens, what's the best way to make an engaging "get out of town" type encounter/session?

Guards in the streets at all hours looking around for the PCs with dogs and torches. In a game full of magic its VERY easy to find people using Scrying and stuff...good time to introduce the head of security for the city and make him a decent level Wizard/Sorceror.

They could get involved with the seedier side of the law to get out (ie. thieves guild etc), or they could try to sneak out on their own (like in the back of a wagon, under supplies or what-not), they could fight their way out (doubtful), or turn themselves in...

As for the cat & mouse with the guards, it should be tough. Guards should have whistles or horns, be in groups of no less than 4 (I'd say at least 8 if the watch is actively searching for them). The dogs are a nice touch, but unless the dogs are modified or the party is only 1st or 2nd level, the group will probably dispatch them easily...

You may have to do some slight rail-roading... The chase is where the action is, maybe in wagons or on horse back or something with hazards all over the place (black smith stalls, fruit stands, children playing in the street, a parade maybe??). Have there be a big fight either before or after the chase, trying to fight AND flee is going to be troublesome. Eventually if they try to stand their ground they will be outnumbered in the city by the guards...

You can set up the chase in a few "scenes" and free-form the in between sections. Meaning, have a few areas where "stuff" happens and rolls have to be made, then just fill in the other parts ad-lib...

The chase set-up will be useful no matter how the decide to flee. When/if they get spotted by the watch, it's "Runner's, on your mark!".

Good luck!

Also, the Head of Security/Watch Captain can become a recurring "villain" if they elude him. It can eventually consume the Captain's life as he vows to hunt down those that murdered his men!

Do a chase scene!

Have them pull a david bruce banner: accused of a crime they did not commit they've been forced to flee. Now the world thinks they're dead and they must continue on, before the monster inside destroys everything they hold dear...

In other words: creep down to the old cemetery, dig up some corpses, burn down a vacant building and put some incriminating evidence within while you sneak away in the confusion.

As for what you can do as the GM: now would be an ideal time for any NPCs you had planned for the party to use long term. The security captain is one angle. Another would be the helpful soul that JUST so happens to have a foolproof scheme out of town. She doesn't need you to pay her back now; just promise to help her out later, when she REALLY needs you.

You could also have an outsider arrive; devils or fey are famous for their "desperation" bargains, but you could also have an angel offer to absolve them and free them from the city if they work as its agents.

Finally you could set it up as an intro to a side adventure. "Quick, down this manhole cover; you'll be safe down there!" Turns out the "friendly" inkeeper is actually a cultist/minion of whatever is using the dungeon under the city. He was telling the truth that there's a way out down there. He just wasn't very specific as to what you'd have to GO through to find it...

Dogs can work, they're level three. They actually are involved with some underground thieves (that's where the problem started) so they might be able to get some aid out of the city. I'm getting some good ideas now.
What else?