The Republic of Congo's main opposition party on Monday charged that the legislative election run-off was marred by fraud and irregularities.

"Yesterday's vote witnessed the same fraud and irregularities that marked the previous round of voting" on July 15, Martin Kimpo, national secretary of the Pan-African Union for Social Democracy (UPADS), told AFP.

He accused the ruling Congolese Labour Party (PCT) of robbing his party of as many seats as possible to prevent it from reaching the threshold for a parliamentary group.

"The regime will have a free hand to achieve its goal of amending the constitution in order to allow (President) Denis Sassou Nguesso to run again in 2016," he said.

In the July 15 first round, the PCT took 57 of the 69 seats that were won outright, with another 10 going to its allies and just one to the opposition and another to an independent candidate.

Only 67 seats in the 136-seat assembly remained to be decided Sunday.

The oil-rich central African country has been open to multiparty politics since 1991 but wracked by two civil wars in which Sassou Nguesso, an army colonel who first came to power in 1979, played a prominent role.

Sassou Nguesso, 68, has ruled the country for 28 years already.

The electoral commission said Sunday it had to go through many incident reports and would release results within three days.