Heavy patrols of armoured vehicles and French soldiers could be seen in Bangui, where more than 1,000 people are believed to have been killed in three weeks of unrest.
The Red Cross has said it recovered some 100 bodies in Bangui, including 20 in a mass grave, amid increased fighting this week between Christians and Muslims.
A combined force of 1,600 French troops and 4,000 African union soldiers are struggling to contain the violence, which has wracked the majority Christian country since a March coup by the mostly Muslim Seleka rebels who installed Michel Djotodia as president.
Last week, France's Foreign Minister Laurent Fabius announced that France, which has led the military intervention, would "soon" have military support from other European countries, however he did not specify which countries would send troops.
Automatic gunfire rattled central Bangui and tracer fire marked the sky overnight on Friday, and shots could be heard form the presidential palace and a nearby military camp.
Presidential spokesman Guy-Simplice Kodegue said the shooting was the result of a misunderstanding between Seleka fighters and troops from the African MISCA force.
A source with MISCA said two Congolese police had been killed in the shooting but provided no further details.
Two Chadian citizens were also wounded by stray gunfire at their nearby embassy, a diplomatic source said.
Confusion reigned in the capital, where heavy fighting sparked panic on Wednesday and sent thousands fleeing for shelter at the airport, the base for French and African forces.
"I'm not even trying to understand what's happening," a shopkeeper in central Bangui said.
"The shooting starts, I hide. The shooting stops, I come out. Then it starts again and I hide again."
At least one civilian was killed and several children wounded Friday when Chadian soldiers threw grenades into a crowd, military and humanitarian sources said.
A military source said the soldiers had been faced with angry demonstrators gathered along a road as they drove by with escaping Chadians.
Soldiers from mainly Muslim Chad have been accused of siding with Seleka forces and the AU has said they will redeploy outside the capital to avoid tensions.
The accusations have been fanned by several incidents, including one on Monday when Burundian troops in the AU force said Chadian soldiers opened fire on them as they were disarming former rebels.
The same day, Chadian peacekeepers fired on a stone-throwing crowd of mostly Christian protesters, killing one man and wounding around 40 more.
Much of the unrest has been blamed on Christian militias carrying out revenge attacks on Muslims after months of abuses by ex-Seleka rebels.
Some 600 French troops were deployed in two flashpoint neighbourhoods on Thursday in a bid to prevent further violence.
French troops and African Union soldiers are trying to restore order in the chronically unstable country after receiving a UN mandate in early December.
The violence is estimated to have forced more than 700,000 people from their homes across the country — including more than 200,000 in Bangui alone.
The country's top Muslim and Catholic clerics have jointly called for extra peacekeepers to be deployed, while US Secretary of State John Kerry said Washington is "alarmed" by the rise in fighting.