"They are there simply as a precaution in case it is necessary to protect French citizens in Kinshasa," one diplomat said.
Diplomats said that a company of French troops flew to the Congolese capital, Brazzaville, last Monday from a base in Gabon, one of France's former African colonies, where French troops are based under defence accords. A company normally numbers about 120 men.
No official comment was immediately available from French embassies in Kinshasa or Brazzaville. The twin cities are separated by the River Zaire.
Tension has been mounting in Kinshasa over the past two weeks since news struck home of a humiliating defeat of Zairean soldiers by Rwandan- backed ethnic Tutsi rebels in the east of the country.
Rebellious students allied to the radical opposition have been demonstrating in a campaign to topple the Prime Minister, Kengo wa Dondo, who is part Tutsi and whose government is accused of doing too little in the face of the rebellion.
The French troops arrived on the eve of a deadline yesterday set by the students for Mr Kengo's resignation, but a student leader said that the ultimatum had been deferred until further notice.
Last Monday students set up roadblocks on Kinshasa streets and hijacked vehicles, bringing transport and business in the city of 5 million people to a standstill. Shops were shut, as many people feared a bout of looting.
French troops intervened in Kinshasa in 1991 and 1993 to evacuate French citizens and other Europeans after army-led looting in which hundreds of Zaireans died. The French ambassador was shot dead in the rioting in 1993.
Tension was close to boiling point last week after two students were killed in clashes with security forces.
Angry students briefly occupied parliament and paraded the coffins of their fallen comrades. Authorities reacted on Sunday by shutting two flashpoint campuses in Kinshasa. Troops drove out students at gunpoint before dawn on Monday. A leading student activist, Steve Mbikayi, said yesterday that the suspension of the deadline for Mr Kengo's resignation was tactical.
"We are waiting to negotiate with the government to reopen two colleges that were shut down at the weekend. When they were kicked off the campuses, many of the students had no homes to go to. Once they are back on campus we will continue the struggle," Mr Mbikayi said.