The reinforcements increased the French military presence to about 1,200 soldiers, who pitched in with United Nations peacekeepers to prevent skirmishes between Muslim and Christian militias.
"We are 24 hours ahead of schedule with the total deployment," a defense ministry official told the BBC after 200 French soldiers arrived in the capital Bangui from neighboring Cameroon.
The fresh troops patrolled the streets of the city and moved into positions on the northern and western approaches, Radio France Internationale said.
The Central African Republic has been in turmoil for months, with Christian militias loyal to ousted President Francois Bozize clashing with Muslim groups backing Michel Djotodia, who installed himself as the country's leader in March.
The French received a warm welcome from the largely Christian population of Bangui, the BBC said, while the Muslim militias withdrew from the city before their arrival.