A female Canadian journalist reporting for the Associated Press (AP) news agency was beaten to the ground, apparently after being mistaken for an American citizen.
“You whites are the ones messing us up,” shouted an angry man in the crowd, heard by AFP.
The journalist was treated at a nearby United Nations clinic and later released.
The United States — South Sudan’s biggest aid provider — announced restrictions on arms last Friday, impatient after more than four years of war that has killed tens of thousands, forced millions from their homes and triggered outbreaks of famine.
Washington has urged South Sudan’s neighbors and other countries to take similar measures.
Protesters claimed the US move was a tactic aimed at undermining the government.
“We are demanding the government of the United States of America review this decision,” said Taban Luka, one of the organisers.
The crowd marched with banners to the US embassy to hand over a petition, and then continued to the main base of the UN Mission in South Sudan (UNMISS) where some threw stones.
One man yelled at the compound, “You UN are even the ones who brought all this suffering. Get out of South Sudan!”
South Sudan gained independence from Sudan in 2011, the result of a 2005 agreement that brought the curtain down on Africa’s longest-running civil war.
But little more than two years later, conflict erupted again when President Salva Kiir accused his former deputy and political rival Riek Machar of planning a coup.
As the violence rapidly spread among the ethnically diverse nation, the UN opened its gates to take in thousands of terrified civilians, frightened for their lives.
Successive rounds of peace talks hosted in neighbouring Ethiopia have failed to end the conflict.
International mediators blame both government and the rebels for perpetuating the war with no regard for the suffering.
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