Assuming the generalized Riemann hypothesis, we provide explicit upper bounds for moduli of logL(s) and L'(s)/L(s) in the neighbourhood of the 1-line when L(s) are the Riemann, Dirichlet and Dedekind zeta-functions. To do this, we generalize Littlewood's wellknown conditional result to functions in the Selberg class with a polynomial Euler product, for which we also establish a suitable convexity estimate. As an application, we provide conditional and effective estimates for the Mertens function.