We present the design, theory and experimental implementation of a low modal volume microlaser based on a line-defect 2D-photonic crystal waveguide. The lateral confinement of low-group velocity modes is controlled by the post-processing of 1 to 3microm wide PMMA strips on top of two dimensional photonic crystal waveguides. Modal volume around 1.3 (lambda/n)(3) can be achieved using this scheme. We use this concept to fabricate microlaser devices from an InP-based heterostructure including InAs(0.65)P(0.35) quantum wells emitting around 1550nm and bonded onto a fused silica wafer. We observe stable, room-temperature laser operation with an effective lasing threshold around 0.5mW.