We present a study on the magnetic properties of manganite ultrafine grains assembled in nanotubes of 800 nm of external diameter and 6-8 µm length. The study includes two homogeneous ferromagnetic compounds, La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃, and one exhibiting ferromagnetic and charge-ordered phase coexistence, La_0.325Pr_0.300Ca_0.375MnO₃. From magnetic measurements, we conclude that the grains behave as single magnetic domains. Observations of dipolar interactions between magnetic grains are evidenced by isothermal remanent magnetization and direct current demagnetization experiments. These experiments suggest that the grain magnetic moments should be arranged in a fanning configuration at H=0. Also, a uniaxial shape anisotropy was observed on previously aligned ferromagnetic nanotubes by ferromagnetic-resonance experiences. In La_0.325Pr_0.300Ca_0.375MnO₃, we observe distinctive features related to its inhomogeneous character as compared with the ferromagnetic homogeneous nanotubes.