There are two 24-hour hospitals in Montreal. The closest one is the Centre Vétérinaire DMV in LaChine, near the airport. They are roughly 13 kilometers from the Plateau, about a 20-25 minute drive assuming no traffic.
Their emergency phone number is (514) 6333-8888.
The other one is the Hôpital Vétérinaire Rive-Sud in Brossard, about 20 km away (a 30 minute drive, barring traffic).
Their phone number is (450) 656-3660.
Those are really your only options if something happens outside of normal veterinary hours.
There is also an animal ambulance service called K911 Transport. These guys come out of Verdun and charge upwards from $89 for coming out. Prices can go up depending on the type of service given. Note that many cabs may not allow you to put an injured animal in their car, so this service could be well worth the price if you are not a vehicle owner.
Their phone number is (514) 677-4357. You may have to leave a message, but they guarantee they'll call back within 5 minutes.
I'd like to thank the woman who answered the phone at the Chaton Santé Clinique Vétérinaire (an excellent cat-only vets that we use), who was very helpful in providing me with the info I needed.
I'd also like to thank Wayne at K911 Transport, who answered my emails promptly and fully. He gave some very helpful general advice for how to behave when coming upon a situation with an injured animal and its owners:
We all try to do what we can when an animal is injured, the fact that you were there and felt for the dog and its owners believe it or not is enough when you're a bystander. The owners have to grieve the process of losing their pet to mourning the loss and it's best not to approach as some people get really offensive having strangers hover over, even if it's just to say you're sorry for their loss .. they just need space. It's never easy and our job is not easy as well...
In an emergency situation its always best not to panic as your pet feels your energy thus adding to his injuries..even though it can be life threatening... easier said than done I know I been there myself.
Excellent advice all around.
I'm keeping those phone numbers in my wallet to have them handy.