Burundi's opposition on Thursday filed a petition to the country's constitutional court in a bid to invalidate the result of last week's referendum, which is expected to strengthen President Pierre Nkurunziza's hold on power.Seventy-three percent of voters cast their ballot for "Yes" in the May 17 referendum to change the constitution, in a move that could give Nkurunziza the option to stay in power until 2034.Even before the results were tallied, the Amizero y'Abarundi (Burundians' Hope) coalition that campaigned for a "No" vote had rejected the outcome, condemning vote-rigging and intimidation."We have just filed our complaint to request that the results of the May 17, 2018 referendum be annulled," spokesman for the parliamentary group, Pierre-Celestin Ndikumana, told the media."There was a lot of intimidation, arrests, with people prevented from participating in our campaign meetings," Ndikumana added."The vote was not free because people were accompanying voters in the polling booth."* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookObservers had widely expected the reforms to pass, partly due to support Nkurunziza still holds in rural areas, but also due to a three-year crackdown on dissent, the media and civil society.The United States has denounced the "climate of fear and intimidation" and "lack of transparency" it said had marred the referendum and questioned the results.Nkurunziza, 54, who has been in power since 2005, plunged his tiny east African nation into crisis in 2015 when he circumvented a constitutional two-term limit, arguing his first term came after an election by parliament.At least 1,200 people have died and 400,000 displaced in a crackdown on protests against his rule, according to the International Criminal Court.The constitutional reforms, which include measures that hand more power to Nkurunziza and his ruling CNDD-FDD, change term limits to seven years, meaning he could start again from scratch in 2020.Nkurunziza is the latest African leader to seek changes to the constitution in order to stay in power, along with Rwanda's Paul Kagame, Uganda's Yoweri Museveni, Cameroon's Paul Biya among others.