Protesters, including women and children, gathered in a football stadium in Mogadishu, the capital, on Monday chanting anti-American slogans and accusing Ethiopia of planning to invade.
Abdirahman Janaqow, a leader of the Union of Islamic Courts, told the crowd: "They say this is a peacekeeping force ... but we see this as an attack on us."
Last week, Washington proposed a resolution at the UN Security Council to deploy east African peacekeepers to Somalia.
The move was requested two years ago by Somalia's interim government.
Güncelleme Tarihi: 20 Eylül 2018, 18:16
Some in Somalia believe a peacekeeping force would provide cover for Ethiopian troops, allowing them to undertake an all-out assault on the Union of Islamic Courts.
The Islamic Courts, which took control of Mogadishu in June and has introduced Sharia (Islamic law) across most of southern Somalia, has been outspoken in the past over the plans to send foreign troops to the country, condemning the resolution as part of a US-led war against Muslims.
Washington's draft resolution also proposes to ease a 14-year-old arms embargo on Somalia. This would let peacekeepers bring in their own weapons and train and equip local security forces.
The Islamic Courts says that if this happens, it will invite fighters from other Muslim countries to come to fight for its cause.
Sheik Hassan Dahir Aweys, an Islamic Courts leader, said at the protest on Monday that the UN "must stop its clear aggression and bias, if not, I swear in the name of God, Muslims as one body will defend themselves".
The Islamic Courts is the country's most powerful armed group and a peacekeeping force might give the interim government some weight in its negotiations with them.
Many critics, however, believe it could just as easily bring the country closer to civil war which might spread into a regional conflict drawing in Ethiopia and Eritrea.