Burundi's main opposition coalition has asked the constitutional court to invalidate the results of last week's referendum on constitutional changes to allow the president to stay in power until 2034.

The leader of Amizero y'Abarundi's parliamentary group said Thursday that the vote was marred by intimidation and abuses.

Pierre Celestin Ndikumana told reporters they were optimistic "considering the extent of the complaints."

The election commission has said a majority of voters approved the changes.

The constitutional court still has five days to validate the results of the May 17 referendum, which the opposition denounced as an effort by President Pierre Nkurunziza to cling to power.

The court three years ago validated Nkurunziza's third term, which the opposition and many international observers saw as unconstitutional. Months of deadly political turmoil followed.