Gunmen opened fire outside Congo's Supreme Court in Kinshasa on Tuesday during a protest by supporters of presidential candidate Jean-Pierre Bemba, an agency witness said.
Bemba at the weekend filed a formal challenge to the court against a provisional presidential election result giving victory to his rival, incumbent President Joseph Kabila.
He has alleged "systematic cheating" in the October 29 run-off vote in Democratic Republic of Congo.
Police fired tear gas at several hundred Bemba supporters who were protesting outside the Supreme Court building.
Then automatic rifle fire broke out and continued in sustained bursts, a Reuters journalist at the scene said.
Black-robed judges and civilian staff fled from the court building as the firing broke out.
A police truck was burning outside the court.
The Reuters journalist saw fighters loyal to Bemba with sub-machine guns at the scene.
United Nations and European Union peacekeepers had protected the October 29 vote, which went off generally peacefully in the vast, former Belgian colony, although supporters of Bemba and Kabila fought gunbattles in Kinshasa earlier this month and in August.