Riot police beat protesters with batons and fired stun grenades which knocked several people unconscious, a Reuters reporter said. Red Cross officials carried at least two people away.
Demonstrators gave out pamphlets quoting 19 presidential candidates who have called for the July 30 election to be postponed.
The candidates want five million spare ballot papers to be destroyed to prevent them being used to fix the poll results. The electoral commission says it needs the spare ballots in case any others are destroyed by accident.
Police said Tuesday's demonstration was illegal. The interior ministry has said it will allow public protests - permitted under a new constitution overwhelmingly approved by a referendum last December - if it receives three days' notice.
Protesters tore down the election
posters of the current president
Protesters included some supporters of the UDPS opposition party, which is boycotting the elections. They tore down election campaign posters and chanted slogans against the current president, Joseph Kabila.
Opponents say Kabila is favoured by foreign governments overseeing the elections.
Pamphlets distributed by demonstrators read: "Congolese brothers, open your eyes and rise up. They want rigged elections to make sure their candidate wins."
Some of the protesters shouted insults at foreign reporters, accusing them of bias.
The poll will be the first free election in the Democratic Republic of Congo for 40 years.
Four million people are estimated to have died in Congo's 1998-2003 war, and the subsequent militia violence.