Unidentified gunmen opened fire on the rally near Rutshuru in the North Kivu province of the country reviving fears that the polls could be disrupted by violence.
The central African country will hold its first free multi-party election in four decades on July 30, but rebels and militias continue to terrorise the civilian population despite the presence of the world's biggest United Nations peacekeeping force.
Jean-Luc Mutokambale, the independent parliamentary candidate who staged the rally, fled to Uganda in fear for his life after the shooting. Other local candidates have asked for UN protection.
Officials said on Tuesday that several people were also wounded in the worst violence so far.
Jacqueline Chenard, a UN spokeswoman in North Kivu, said a team had been sent to investigate the attack.
"Many candidates in these areas cannot carry out their campaigns freely. The situation is precarious just two weeks before the elections," she said.
In the capital, Kinshasa, several hundred people protested at alleged irregularities in the electoral process before police used tear gas to break them up. Election posters were torn down during the demonstration.
Hundeds protested against
irregularities in the poll process
International donors backing the election wanted the country's army to be confined to barracks until the voting is over.
This is necessary "to maintain a climate of calm during the vote and also to ensure the army is not politicised," the donors' group said in a statement.
The group, which includes the UN, said that Congolese police and UN peacekeepers should maintain law and order during the polls.
Demobilised rebels and militia fighters are being integrated into the fledgling army and human rights groups accuse them of regularly killing, raping and robbing civilians.
The election is intended to start a new era of stability in Congo after a 1998-2003 war which involved six of the country's neighbours and left about four million people dead through violence, hunger and disease.
Congo has the world's biggest UN
Despite the Democratic Republic of Congo's vast natural resources, the five -year conflict created a humanitarian crisis as all sides used the situation to plunder them.
President Joseph Kabila - who took power after his father was assassinated in 2001 - is standing against 32 other contenders in the July 30 polls.
Some presidential and parliamentary candidates have called for a suspension of the campaign over fears that election authorities may rig the vote.
Kabila's opponents say the international community is clearly backing the current president and accuse him of abusing his control over state media and the security services to gain an unfair advantage over other contenders.
25 million voters are expected to cast their ballots at 50,000 polling stations across the former Belgian colony.