The prime minister of Somalia's UN-backed government, Omar Sharmarke, described it as a "vicious and calculated outrage".
The African Union condemned the bombing - at a graduation ceremony for medical students - as "inhumane and cowardly". No group has claimed responsibility for the attack, in the capital Mogadishu.
Somalia's President Sheikh Sharif Sheikh Ahmed described the attack at the Shamo Hotel as a "national disaster".
The African Union said the blast would "not deter the resolve and determination of the African Union to support the people of Somalia in their quest for peace and reconciliation".
The acting head of the AU's peacekeeping force Amisom, Wafula Wamunyini, said the blast was "intended to intimidate and blackmail" the UN-backed government.
UN Secretary-General Ban Ki-moon said in a statement the bombing marred "what should have been an event filled with hope for Somalia."
A statement signed by the UN, the US, the EU and the Arab League said the attack would not deter the international community from continuing its support to the Somali government.
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