We present the results of a lattice QCD calculation of the average up- down and strange quark masses and of the light meson pseudoscalar decay constants with N-f = 2 dynamical fermions. The simulation is carried out at a single value of the lattice spacing with the twisted mass fermionic action at maximal twist, which guarantees automatic O( a)improvement of the physical quantities. Quark masses are renormalized by implementing the non perturbative RI- MOM renormalization procedure. Our results for the light quark masses are m(ud)((MS) over bar) (2 GeV) = 3.85 +/- 0.12 +/- 0.40MeV, m(s)((MS) over bar) ( 2 GeV) = 105 +/- 3 +/- 9MeV and m(s)/m(ud) = 27.3 +/- 0.3 +/- 1.2. We also obtain f(K) = 161.7 +/- 1.2 +/- 3.1MeV and the ratio f(K)/f(pi) = 1.227 +/- 0.009 +/- 0.024. From this ratio, by using the experimental determination of Gamma(K -> mu(nu) over bar (mu)(gamma))/ Gamma(pi -> mu(nu) over bar (mu)(gamma)) and the average value of vertical bar V-ud vertical bar from nuclear beta decays, we obtain vertical bar V-us vertical bar = 0.2192( 5)( 45), in agreement with the determination from K-l3 decays and the unitarity constraint.