France condemned the attack and said police were investigating.
The 76-year-old senator, Leon Kengo, was attacked at the Gare du Nord train station on Saturday "by bands of those who call themselves 'fighters' close to Etienne Tshisekedi", Communications Minister Lambert Mende told AFP.
Tshisekedi is a veteran opposition leader in the Democratic Republic of Congo and runner-up in the November 28th election but he has refused to recognise Kabila's victory and has proclaimed himself the country's leader.
According to initial reports, Kengo "had some teeth knocked out, was trampled underfoot, and rolled on the ground", Mende said, adding that the lawmaker had been hospitalised in north Paris.
He had got off a train from Brussels and was getting into a waiting car when he was brutally attacked, a source close to the investigation said.
"We protest in the strongest terms this attack perpetrated in France against Congolese officials in the view and knowledge of French authorities," said Mende, calling it "totally unacceptable".
He claimed that Kengo, who came in fourth in the presidential poll with 4.95 percent of the vote, was attacked because Tshisekedi supporters were angry that he had not withdrawn his candidacy, which may have prevented their leader from being elected.
The Congolese foreign minister called in the French ambassador "because this incident is very serious and worrying to the Congolese government", Mende said.
"We regret this attack," French foreign ministry spokesman Bernard Valero told AFP.
"Security forces naturally came to his assistance as quickly as possible and French authorities ensured that he was admitted without delay to a Parisian hospital to be treated," Valero said.
"An investigation is underway," he added.
Kengo was one of the presidential candidates who had called for annulling the vote over fraud allegations. Several foreign countries and election observer missions also complained of irregularities in the vote.
Still Kengo was present at the swearing-in of Kabila to a second five-year term on December 20th.
The French ambassador to Kinshasa, Luc Hallade, told AFP he denounced these "rogue methods", adding: "this is not how can advance democracy in Congo or anywhere else".