How much force is needed to tow an airliner? Airliners are often (almost always actually) pushed back by a tug or a tractor to leave the gate. The tug can also tow an aircraft that cannot taxi by itself.

How much force is required to pull or push a large airliner on the ground? In other words, what is the horsepower of these tugs?

EDIT: By large airliner, I mean Boeing 747 / 777 / A380 type, the ones which ATC append "Heavy" (or "Super") to their call sign.
Tugs do not need that much horsepower, even the biggest tugs only have around 400bhp. What they do need is lots of torque for push/pull power, lots of weight to give it plenty of inertia, and big grippy tires to transfer the power to the surface. 

The T80 in this link is pretty typical, 50-70 tons, 408bhp, and 1500 pound feet of torque. That's more than triple the torque of a hefty pickup.