United Nations Security Council envoys have met Democratic Republic of Congo President Joseph Kabila to urge calm in the first multiparty polls in 40 years.
The UN is helping the army flush out rebels in the east
They expressed their concern about ultra nationalist speeches some politicians are making in campaigning.
The UN considers the vote, scheduled for late July, crucial to regional security, and is deploying 18,000 peacekeeping troops to maintain order.
Meanwhile, police have fired tear gas at opposition protesters in Kinshasa.
Several hundreds supporters of two opposition parties, the UDPS and Fonus, marched through the capital but some started to throw stones before police took action.
They say the government failed to consult widely enough when it drew up the timetable for the poll.
The BBC's Arnaud Zajtman in Kinshasa says many Congolese do not believe that the poll will be free.
Meanwhile, Vice-President Azarias Ruberwa has expressed doubts that the elections will be held on 30 July, as planned.
"I have serious reservations on the chances of all the ballots arriving at their destinations on time," said Mr Ruberwa, a former rebel leader who is a presidential candidate, according to the AFP news agency.
Incumbent Joseph Kabila is favourite for the presidential poll
The EU has approved a 2,000-strong rapid reaction force during the polls.
The 15-strong UN delegation, which arrived on Saturday night, met President Kabila and the country's four vice-presidents, who represent rival former rebel movements in the interim government.
The head of the mission, French UN ambassador Jean Marc de la Sabliere, said they expressed their concern to Mr Kabila that politicians have been questioning whether their opponents are Congolese.
"He [President Kabila] thinks the whole thing should calm down and that this campaign could be dangerous."
The delegation was also scheduled to meet political and economic leaders as well as members of the independent electoral commission and representatives of the international community, the AFP news agency reports.
Mr Sabliere said stability in DR Congo was "vital for the Great Lakes region and beyond that, for the African continent as a whole".
On Sunday, the head of DR Congo's media authority asked the UN envoys to demand an end to what he described as incitement to hatred by the country's media.
"If we're not very careful, the whole campaign will assume this character of intolerance and incitation to hatred," Modeste Mutinga, president of the High Authority of the Media warned.
In other African countries, such as Ivory Coast and Rwanda, media hate-campaigns like this have led to war and genocide.
The EU rapid reaction force will include more than 400 troops in Kinshasa, while the rest will be based in neighbouring Gabon and in Europe, and will intervene only if asked to do so by the UN.
France and Germany will be the main contributors, sending about 800 soldiers each, but more than 10 other countries are sending troops too.
Rebels remain active in the east of the country, presenting a threat to free elections.
Seven Nepalese UN peacekeepers are still being held by rebels in the east of the country after being captured two weeks ago.
Although tension is rising ahead of the polls, most of the country is relatively calm, except for the east and the south-eastern Katanga province.