Spokesman Maj. Barigye Bahoku says insurgent groups have prepared several car bombs for the attacks. He urged the public Wednesday to be on the lookout for ten vehicles, some of them falsely bearing the AU's insignia.
He says rebels have become desperate after suffering from infighting and a drop in public support.
More than 30 worshippers were killed in two blasts inside a mosque in the capital on Saturday. Warring parties in Somalia have said there will be more attacks.
Somalia has not had an effective government for 19 years.