The UN mission in the Democratic Republic of Congo is investigating the violent dispersal of a demonstration in the capital, Kinshasa, on Tuesday.
Riot police beat some protesters with batons and fired tear gas
Their inquiry is to try to establish whether the demonstrators' number or behaviour justified the use of force by the Congolese police, the UN said.
Several people were injured in the clashes after election protests.
The 30 July polls are supposed to be the first free elections in 45 years.
Observers have also complained about irregularities in the election process.
"Government actors have deliberately attempted to intimidate and obstruct certain candidates in their campaigning," said the US-based Carter Center.
The organisation set up by former US President Jimmy Carter also said that customs officials were not treating imported election material equally.
DR CONGO'S ELECTIONS
33 presidential candidates
8,650 parliamentary candidates
500 parliamentary seats
267 registered political parties
25m registered voters
50,000 polling stations
A UN spokesman confirmed that there had been some problems but said he was generally "satisfied" with the campaigning.
Three radio stations in central DR Congo have been closed down for broadcasting "unbalanced" election reports.
The authorities say the radio stations - two in Equateur province and one in Kasai province - were creating division because of the extent to which some candidates are being attacked on air.
The UN, backed up by 17,000 peacekeepers, is helping to organise the polls, which are intended to end a power-sharing period following the end of the five-year civil war.
At Tuesday's rally in the centre of Kinshasa, the BBC's Arnaud Zajtman saw one protester with badly wounded hands, following an incident involving a tear gas canister.
He also saw an unconscious opposition MP being taken to hospital.
The protesters say they informed the authorities about the demonstration, but despite this the police attacked the rally with batons and stun grenades, leaving several people unconscious.
The march was held by supporters of 19 of the 33 presidential candidates who have not started campaigning because they are unhappy at the process.
They say five million spare ballots could be used to rig the elections.
Some have also complained about state media being biased in favour of President Kabila.
On Tuesday, 10 people were sentenced to five years in jail for electoral fraud in the eastern city of Kisangani,
Another 15,000 people are suspected of registering twice for the polls.
Campaigning in the east has been hampered by instability as the UN and Congolese army try disarm rebels ahead of the polls.
President Kabila has the best financed campaign
In many parts candidates need to get the permission of the militia in charge to campaign in areas under their control.
After years of conflict and misrule, holding the vote will be a major challenge - there are no roads or railways linking one side of the country to the other.
President Joseph Kabila is seen as the favourite to win the polls.
Three former rebel leaders and vice-presidents are also competing.