BUJUMBURA (Reuters) - Burundi’s opposition asked the constitutional court on Thursday to nullify a referendum that could allow President Pierre Nkurunziza to stay in power until 2034, saying the vote was marred by fraud.

FILE PHOTO: Burundi's President Pierre Nkurunziza waves as he arrives for the celebrations to mark Burundi's 55th anniversary of the independence at the Prince Louis Rwagasore stadium in Bujumbura, Burundi July 1, 2017. REUTERS/Evrard Ngendakumana/File Photo

Election officials said 73 percent of voters in the East African country backed the changes in Monday’s plebiscite.

But Pierre Célestin Ndikumana, a lawmaker from the opposition Amizero Coalition, said it wanted to challenge how the vote was held and “the climate that prevailed on the polling day”. “There has been an electoral fraud,” he said.

Rights groups say security forces and their allies in the Imbonerakure youth militia created a climate of fear and intimidation during the campaigning.

A government spokesperson was not immediately available for comment.

Burundi has been wracked by violence since early 2015 when President Nkurunziza said he would seek a third term - a move that critics at the time said would break limits set out in the constitution.

Clashes between security forces and rebels left hundreds dead and forced about half a million to flee - rattling a region still haunted by the memories of the 1994 genocide in neighboring Rwanda, which has a similar ethnic mix to Burundi.

This week’s referendum changed the constitution in a way that would allow Nkurunziza to run for another two terms. It also extended a president’s term from five to seven years.

New York-based Human Rights Watch says at least 15 people were killed while six were raped during the referendum campaign.

Broadcasts by the BBC and the Voice of America were also banned two weeks before the vote.