DR Congo immigration police briefly detained the number two in opposition leader Etienne Tshisekedi's party as he prepared to board a flight to Belgium, security sources said Wednesday.

Jacquemain Shabani, secretary general of the Union for Democracy and Social Progress (UDPS), was stopped with a passport that did not belong to him and "a lot of unnecessarily subversive and offensive documents," an official close to the intelligence services told AFP.

He was detained late Tuesday and released early Wednesday, the official said on condition of anonymity.

"There was a passport he was carrying for somebody else which he did not declare. As for the documents, there was an internal report, with pictures, on the electoral process and human rights violations," a UPDS official said.

Presidential and legislative elections were held in the vast conflict-prone central African nation on November 28 but the results were disputed by the UDPS, plunging the Democratic Republic of Congo in a political crisis.

Official results returned President Joseph Kabila for another term but Tshisekedi said the vote was fixed and declared himself the country's lawfully elected leader.

The veteran opposition leader also dismissed legislative results that gave Kabila and his allies an absolute, albeit reduced, majority in parliament and vowed his party -- the second largest group -- would boycott the house.

Shabani vociferously defended Tshisekedi's cause before, during and after the unrest-plagued vote.

The international community largely snubbed Kabila's inauguration and has condemned widespread irregularities during the electoral process.