At least 10 opposition protesters have been injured as police fired tear gas to break up a rally in the Democratic Republic of Congo's capital, Kinshasa.
Riot police beat some protesters with batons and fired tear gas
Supporters of 19 of the 33 presidential candidates are calling for a delay in the 30 July elections and demanding the destruction of 5m extra ballot papers.
In the eastern city of Kisangani, 10 people have been sentenced to five years in jail for electoral fraud.
Another 15,000 people are suspected of registering twice for the polls.
At the 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 19 candidates have still to begin campaigning. Their supporters say the elections are badly organised and suspect fraud.
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
"Too many ballot papers have been printed. The population demands that they be destroyed," said a male protester.
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.
Meanwhile, the first contingent of German soldiers has arrived in DR Congo, to serve with a European Union force of about 2,000 troops, charged with supervising the landmark polls following five years of conflict.
The EU soldiers - most of whom come from Germany and France - will back 17,000 existing United Nations peacekeepers, the world's largest peacekeeping force.
The BBC's Jonathan Kacelewa in Bukavu says the group sentenced by a tribunal on Tuesday morning were among a group of 18 who have been arrested for the fraud.
The world's largest peacekeeping force is in DR Congo for the polls
But he says there is anger that only a small number of some 15,000 suspected by the Independent Electoral Commission of registering twice have so far been charged.
The UN, which is helping to organise the polls, hopes they will be DR Congo's first democratic elections in 45 years.
As the German troops left for DR Congo, UN Secretary General Kofi Annan praised the EU mission and called on the EU to stay in DR Congo after the elections until stability was fully established.
He said the international community must not repeat the mistakes made in other post-conflict situations.
"Too often, the result has been that a hard-won peace fell apart, and the international community has had to return a few years later. That is a tragedy for all concerned," AFP news agency quotes him as saying.
Campaigning in the east has been hampered by instability as the UN and Congolese army try disarm rebels ahead of the polls.
In many parts candidates need to get the permission of the militia in charge to campaign in areas under their control.
Recently, some 5,000 people have fled their homes in the north-eastern Ituri province.
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.