I would like to drive the car to the
nearest qualified service, but there
is a fair distance to it (about 8km)
as I'm living in a remote location. Is
it safe to drive it that distance if I
refill the entire coolant tank?
I would say yes, you can drive the car. Load the car with much more water than you think necessary: I find that two liter soda bottles make excellent transport and pouring vessels for this sort of thing. While driving, watch your gauge: if you see a high reading, get out and add water to the overflow tank (never open a hot radiator cap). Repeat as necessary.
What can the cause be? I know that is
impossible to diagnose a car without
seeing it, however, if you have any
idea what could the problem be, I
would really like to hear it.
The water is clearly doing what it is supposed to do: carry heat away from the engine. Unfortunately, something is preventing the heat from leaving the water (e.g., sludge as previously mentioned, fouling in the cooling fins or just impeded airflow) or you have a leak.
I think that your instinct to go straight to a service station is a great idea.
If, however, you were to choose to try to diagnose this situation yourself (which I'm not recommending if you're not feeling confident), you could try running the car up to temperature, turning it off and listening and looking for a pressure leak (hissing and / or white steam).
For clarity, never never never feel around for a steam leak with your bare hands. Best case: you'll get a terrible burn.
Following up on that last paragraph to add this lovely quote from Dan:
Clouds, and the visible "steam" squirting out of a kettle or a steam
locomotive, are liquid water droplets with a ceiling temperature of
100°C at sea-level air pressure. It's possible for actual
invisible-vapour steam to be swirled in with condensed droplets as it
mixes more or less chaotically with the outside air, but "pure" steam
is invisible, and has no ceiling temperature. Put your hand in the
visible portion of the steam coming out of the side of a locomotive
and you may get scalded, but putting your hand in the invisible jet
close to where it's exiting may flense the flesh from your bones.
The key phrase there is "no ceiling temperature."