Al-Shabab held rallies in Beledweyne and three other towns Wednesday, where speakers denounced East African nations that have pledged to send an additional 2,000 troops to Somalia.
Witnesses say militants went door-to-door and forced people to attend the rallies.
The regional bloc known as IGAD, the Inter-Governmental Authority on Development, pledged the troops on Monday after a plea for help from Somali President Sheikh Sharif Sheikh Ahmed.
Al-Shabab and another insurgent group, Hizbul Islam, are battling the government for control of the capital, Mogadishu.
The government controls only a few areas of the city with the support of a few thousand African Union peacekeepers.
Al-Shabab and Hizbul Islam are trying to topple the government and set up a strict Islamic state.
Some information for this report was provided by AFP and AP.