The magnetic ordering in La2CuO4 is significantly affected by the existence of antisymmetric Dzyaloshinsky-Moriya interactions. We use an effective classical spin Hamiltonian to obtain the full phase diagram at zero temperature, with a magnetic field oriented along an arbitrary direction in the orthorhombic b-c plane. We show that the various transition fields are sensitive functions of the input anisotropies. Using the measured transition fields [Thio et al., Phys. Rev. B 38, 905 (1988); 41, 231 (1990)], we find that the anisotropy in the intralayer interaction is 3.4 0.6 meV and the interlayer interaction is 1.75 0.2 meV. We predict the entire magnetic phase diagram for fields in the b-c plane, including a critical end point, when the magnetic field is applied at an angle of 26.3°with the c axis. We also discuss the qualitative changes expected in the phase diagram if the value of the antisymmetric exchange increases.