We construct new (n, r)-arcs in P G(2, q) by prescribing a group of automorphisms and solving the resulting Diophantine linear system with lattice point enumeration. We can improve the known lower bounds for q = 11, 13, 16, 17, 19 and give the first example of a double blocking set of size n in P G(2, p) with n < 3p and p prime.