A day after police violently broke up an opposition protest over election irregularities, the Carter Centre, a US-based monitoring body, also criticised some candidates for exaggerating marginal issues rather than campaigning seriously for the ballot on July 30.

Congo's poll should be the first free election in more than 40 years and is intended to draw a line under the country's last war, a conflict that killed four million people, mostly from hunger and disease, between 1998 and 2003.

Jimmy Carter, the former US president who founded the centre, said: "Government actors have deliberately attempted to intimidate and obstruct certain candidates in their campaigning."

The centre accused the government of unjustified arrests, intimidation of businesses to stop them working for challengers and unequal customs treatment of election material.

"Such actions are a serious abuse of the powers of government, foment tensions between the parties, and threaten the fairness and equality of the electoral process," it said.