The two included freelance news photographer Philip Montgomery, on assignment for the Mashable website, and a black-clad male protester with a kerchief over his face.
"They were in the street and refused to move back onto the sidewalk," Schron Jackson, a spokeswoman for the St Louis Metropolitan Police Department, told AFP by email.
"They were arrested for impeding the flow of traffic. Both were issued summonses and released."
Ferguson, a black-majority suburb of St Louis, has been the scene of almost nightly demonstrations after a white police officer fatally shot unarmed African-American teenager Michael Brown on August 9.
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"Right now, we are at a moment where, if we don't demand changes to our system -- not just small fixes but long-term and deep structural fixes to, say, the criminal justice system for instance -- I think we are in deep trouble," one protester, college professor David Ragland, told AFP.
Focal point of the protest was the white colonnaded Old Courthouse, where black slave Dred Scott and his wife unsuccessfully sued for his freedom in 1856.
In adjacent St Louis County, which includes Ferguson, police today pursued their investigation into gunshots fired at an otherwise peaceful Ferguson protest on Wednesday, wounding two police officers -- one in the face, the other in the shoulder.
The incident renewed tension in the city where local entrepreneurs met Saturday with Mayor James Knowles, saying business for them has fallen as much as 80 per cent since Brown's killing put Ferguson in the national spotlight.