Kinshasa, Congo ? Troops fired into a crowd and killed 12 protesters in retaliation for the death of a soldier Friday while campaigning began for Congo’s first multiparty elections in more than four decades.

A human rights worker, Christian Malidini, said soldiers opened fire after demonstrators in the western city of Matadi attacked and killed the soldier. He had no further details, and officials in the city 250 miles southwest of Kinshasa, the capital, couldn’t immediately be reached for comment.

The elections will mark the first democratic vote after more than four decades of coups, war and corrupt military rule. The official start of campaigning on Friday coincided with the anniversary of Congo’s independence from Belgium in 1960.

Nearly three dozen candidates are vying for the presidency and thousands for parliament in elections for a government to replace a transitional administration that took over following the country’s 1998-2003 civil war.

Aid groups estimate violence since 1998 has left some 4 million Congolese dead, mostly through strife-related disease or hunger.

Fearing political clashes, the governor of the province that includes the capital has banned all demonstrations in the city.

But groups of young men still took to the otherwise deserted streets Friday, seeking to campaign for candidates. Police scattered the crowds swinging batons and firing guns into the air. Most shops, banks and schools remained closed.

Meanwhile, the U.N. Security Council voted unanimously Friday to extend the mandate of more than 1,100 peacekeepers in the country until Sept. 30. They are among some 17,500 U.N. troops and police in the country, comprising the largest U.N. peacekeeping operation in the world.

The Security Council urged Congo’s transitional government and all parties “to ensure that free, fair and peaceful elections take place” and that the timetable for polls “is scrupulously respected.”

Hundreds of European Union troops also are expected to arrive in the coming weeks.

Logistical and political problems led to repeated delays in the voting. The elections were to have been held last year but have now been set for the end of the month – even though the transitional government’s mandate technically expired on Friday.