Hundreds of supporters of Congo's opposition parties clashed with armed riot police in the capital Friday.  A group of politicians, led by the veteran opposition UDPS (Democratic Union for Social Progress), was trying to hand a document to U.N. peacekeepers outlining their concerns over upcoming elections when the police intervened, firing tear gas and arresting dozens.

Just one day after Congo's president, Joseph Kabila, promulgated a new electoral law and the electoral commission confirmed elections would be held on June 18, offices for registering candidates opened across the country.

But progress towards elections was overshadowed when much of the center of the bustling capital ground to a halt Friday, with opposition supporters and riot police clashing on the main boulevard.

Heavily armed policemen fired dozens of canisters of tear gas, beat demonstrators with batons and arrested at least 40 people during several hours of violence.

By the afternoon, a relative calm had returned but the security services maintained a heavy presence in the middle of town.

The leaders of the march said they were trying to hand a memorandum to the U.N. peacekeeping mission, which is working to keep Congo on track towards elections after a five-year war. 

The memorandum, they said, outlined their concerns over the coming elections and what needs to be done to ensure opposition participation.

Factions from Congo's war, as well as some unarmed opposition parties, have spent the last three years taking part in a transitional government, aimed at reunifying the country and organizing elections.

But other groups, led by the veteran opposition UDPS party, have boycotted much of the process and outlined a series of demands before they will take part in the election, which would be the first free poll in 40 years.

The U.N. peacekeeping mission expressed its regret at Friday's violence, saying it was ready to receive the opposition memo and was urging the security services to exercise restraint.

Congo's conflict has killed four million people, mostly from hunger and disease, leading experts to dub it the world's worst humanitarian crisis since World War II.