We study the Clique problem in classes of intersection graphs of convex sets in the plane. The problem is known to be NP-complete in convex-set intersection graphs and straight-line-segment intersection graphs, but solvable in polynomial time in intersection graphs of homothetic triangles. We extend the latter result by showing that for every convex polygon P with sides parallel to k directions, every nvertex graph which is an intersection graph of homothetic copies of P contains at most n k inclusion-wise maximal cliques. We actually prove this result for a more general class of graphs, the so called k DIR -CONV, which are intersection graphs of convex polygons whose sides are parallel to some fixed k directions. Moreover, we provide some lower bounds on the maximum number of maximal cliques, discuss the complexity of recognizing these classes of graphs and present relationships with other classes of convex-set intersection graphs. Finally, we generalize the upper bound on the number of maximal cliques to intersection graphs of higher-dimensional convex polytopes in Euclidean space.