  We consider the scaling limit of the two-dimensional $q$-state Potts model
for $q\leq 4$. We use the exact scattering theory proposed by Chim and
Zamolodchikov to determine the one and two-kink form factors of the energy,
order and disorder operators in the model. Correlation functions and universal
combinations of critical amplitudes are then computed within the two-kink
approximation in the form factor approach. Very good agreement is found
whenever comparison with exact results is possible. We finally consider the
limit $q\to 1$ which is related to the isotropic percolation problem. Although
this case presents a serious technical difficulty, we predict a value close to
74 for the ratio of the mean cluster size amplitudes above and below the
percolation threshold. Previous estimates for this quantity range from 14 to
220.
