Static and dynamic magnetic properties of P (3) over bar 1m-phase MnSb2O6 have been studied by means of muon-spin relaxation (mu SR), high-frequency electron-spin resonance (HF-ESR), specific heat, and magnetization studies in magnetic fields up to 25 T. The data imply onset of long-range antiferromagnetic order at T-N = 8 K and a spin-flop-like transition at B-SF approximate to 0.7-1T. Below T-N, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below T-N2 = 5.3 K is evidenced by a step in the magnetization and a slight kink of the relaxation rate. Above T-N, both mu SR and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least 12T(N) approximate to 100 K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of Delta approximate to 18 GHz which implies small but finite magnetic anisotropy.