Congolese opposition leader Felix Tshisekedi rejected suggestions that he might do a deal with President Joseph Kabila, at a rally which passed off peacefully on Tuesday.

Tshisekedi is one of two main figures expected to seek to run in a delayed election scheduled for December. Exiled businessman Moise Katumbi is the other.

Tshisekedi dismissed speculation he might accept the post of prime minister, something Kabila has offered in the past to appease opponents.

"There is nothing like any dialogue for a prime ministerial post," Tshisekedi told the cheering crowd.

"The (party) elected me to be presidential candidate, so how can I be lowered to prime minister? Rather, let me be led to the final victory in the presidential election."

Kabila's opposition is weak and divided. Many opposition politicians joined a power-sharing government after the death of opposition leader Etienne Tshisekedi, Felix's father, last year.

Dozens have died in protests, most shot dead by security forces, since Kabila refused to step down at the end of his constitutional mandate in December 2016. But Tuesday's rally was unusual in that it was permitted, watched closely by armed police.

Popular anger could easily boil over if elections are delayed again or are not perceived to be fair. In addition, several parts of the country are dealing with armed rebellions.

Donors have expressed concern about plans to use new electronic voting machines, saying the system is untested and could allow fraud. Congo's government reacted by saying it would reject foreign aid for the poll.

"Their machine is a machine to cheat. The electoral register is corrupt and merits a serious audit," Tshisekedi said.

The electoral commission has denied accusations of bias.