(This is an entire campaign, and begins with 1st level chars that have never met each other.)
Each PC is doing normal, everyday things (sword practice, study, drinking, etc) when he is arrested by the city police (knocked unconcious if they do not go peacefully). The PC's all end up in the same jail cell. The next day, they are brought to trial for the murder of some important official. They are convicted and sentenced to burn tomorrow. They are returned to their cell (stripped of all equipment) to await their execution.
The PC's have at least two escape paths: (more, if they're creative)
Once they are out, they must flee the city (if they try to stay, tell them the police have noticed their escape, and are beginning a house-to- house search. This information could come from a bartender or similar person.) They may wish to steal some equipment, or maybe a friend will provide them with weapons, urging them to run.
The PC's can travel either to the ocean (if they can capture a small vessel) or to the unexplored mountainous regions. There, they can gain experience and hide until they are ready to return, and find out who framed them for the murder. (It was the judge, or maybe another politician. After killing the victim, he planted evidence pointing to the PC's. The PC's may have been political opponents of him, or just randomly chosen.)