Protestors marching against the enslavement of black Africans in Libya blocked traffic in all directions from Parliament Square.
The demonstration jammed roads in Westminster, causing heavy queues on all approaches.
TfL reported that traffic was backed up on Westminster Bridge and Victoria Embankment.
African Lives Matter led the protest as Parliament debated a petition against the reported slave trade on Monday.
A TfL spokesman said there was "serious traffic impact" after crowds descended on Parliament Square.
He added that there is "congestion on all approaches" to the area.
The human rights group said the march was "a peaceful protest to bring attention to the human rights violations taking place present day in Libya".
In statement, African Lives Matter added: "As of now the following atrocities are happening: The enslaving, selling and illegal detention of black Africans in Libya.
"Violence against men, women and children including the extrajudicial killing of children."
A debate on a petition relating to the human rights issue took place in Parliament at 4.30pm, while protesters outside called for African leaders to "immediately repatriate migrants held captive".
The petition, which was launched after CNN released video footage of men in Libya being sold into slavery in October, received nearly 270,000 signatures.
A spokeswoman for Met Police said: "We are aware of demonstrations today on Monday December 18 at Old Palace Yard.
"There is an appropriate policing plan in place.
"Officers are on the scene and are dealing with traffic issues as a result of the demonstration.
"No arrests have been made."
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