Military spokesman Brigadier Udaya Nanayakkara said the injured were rushed to hospital.
Earlier on Sunday a hand grenade exploded in a zoo in the suburb of Dehiwala, injuring seven.
The bombings came as the country prepares to celebrate 60 years of independence from British colonial rule on February 04 and the capital is under a tight security blanket.
Police have blamed the Tamil Tigers for earlier bombings which have targeted civilians.
Sri Lanka pulled out of a ceasefire with the Tamil Tigers, who have been fighting for a separate state in the island for nearly three decades in January.