Along with dodging the many bikes that come from all directions, I noticed that the ancient street stones, especially around the Grote Markt area, are slippery when wet AND really work you legs into submission. You will feel some pain overnite especially if you are not used to walking on this kind of surface.
I think it's okay to claim Leuven is a safe and clean place. There's little crime here - you hardly ever hear about a murder, robbery, rape or assault having happened here.
When walking the streets late at night you may bump into a drunk but that's about it.
Most places are well lit and you'll rarely see groups of people loitering. The picture to the left was taken in the subterranean passage underneath the railway station, in the middle of the night. Stations are usually a good indicator of a city's safety. (I'm sorry to say Leuven station is in stark contrast with places like the Brussels North station area)
The main danger to take into account are forms of theft like pickpockets and getting your bike stolen. So just lock your bike properly and mind your belongings like you would do in any city.
The police station couldn't be easier to locate : it's to the left side of the town hall on the Grote Markt (Great Market Square). The station is open 24/7, the lost and found is also here (both reporting and picking up, between 07:00-22:00) the general phone number of the station is 016 / 21 09 00.
In case of an emergency (i.e. in case you need an amublance, urgent police assistance or the fire deparment), dial 100 (our version of 911) and the dispatch unit will handle your call.
I would advise against driving any motorvehicle in Leuven if you're not familiar with the city, due to the number of one way streets and the fact that the very center of the city is virtually traffic free except for cyclists, buses and taxi's.
If you must drive a car in Leuven, please note that in most of the inner city and on many parts of the ring surrounding Leuven, there's a 50km/h speed limit - and there are many cameras to register offenders. In the inner city, even 30km/h speed limits exist here and there. If you get pulled over and fined for speeding, double parking etc, foreigners must pay the fine on the spot - even if it means being escorted by the police to the nearest BanContact / ATM to collect the cash. I kid you not.
Because Leuven's population is about two-thirds student, and the town core is very small, there is a great market for renting old clunky (but very fun) bikes. Each of these has a solid carrier on the back that acts like a back seat, so a team of bikes can really transport a lot of people around!
Anyways, there are just about as many bikes stolen as there are bikes rented in Leuven. I've met Belgians who steal a bike every night on the way home from the bar, and just leave it out for someone else to steal that night. There's so much bike theft here that it almost feels justified. However, watch your bike, leave it locked, and don't panic if it disappears.
Watch out at the "Old Market", that's where all the cafés are. At night, there's always police at he roads and squares, and if you're drunk and too noisy , they may take you to the bureau and let you pay for "Openly drunkenship" ..
In and around Leuven there are dozens of hidden speed traps. There are 10 on the ringway alone, thats one every half a mile or so! Pay close attention as in Belgium we have the most expensive traffic tickets in the whole world!
Don't be so focussed on one thing that you miss all this city has to offer. In 1988 we were so focussed on the Town Hall that we failed even to notice the Cathedral just opposite, and we certainly didn't notice just how great the nightlife is here (I'd suggest you try to spend some part of an evening in town).