Burundians vote on Thursday in a referendum on constitutional reforms that are set to shore up the power of President Pierre Nkurunziza, enabling him to rule until 2034.The referendum comes as Burundi remains mired in a deep political crisis - triggered by Nkurunziza's run for a controversial third term in 2015 - that has killed 1 200 and forced 400 000 from their homes.Voting in Thursday's straight yes or no ballot is due to begin at 06:00 with polls closing at 16:00.At least 4.8 million people, or a little under half the population, have signed up to vote, according to the Independent National Electoral Commission (CENI), which is running the referendum.The changes will be adopted if more than 50% of cast ballots are in favour.But with opponents cowed, beaten, killed or living in exile, there seems little doubt the amendments will pass, enabling the 54-year-old - in power since 2005 - to remain in charge for another 16 years.Exiled civil rights activists this week called for sanctions against Nkurunziza's regime.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook"Do something - don't just issue statements," Pierre Claver Mbonimpa, a renowned rights defender, urged the international community at a press conference in Paris.The campaign period, like the preceding three years of unrest, has been marked by intimidation and abuse, say human rights groups.The International Federation for Human Rights (FIDH) said there had been "a campaign of terror to force Burundians to vote yes"."The process has not met any of the criteria to ensure credibility. It has been marked by warnings, threats, intimidation and repression," said FIDH's Tcherina Jerolon. 'Death knell' for peace deal The vote is taking place in tightly-controlled conditions, and a presidential decree ruled earlier this month that anyone advising voters to boycott the vote risks up to three years in jail.The tiny central African nation has struggled to recover from a brutal and destructive civil war from 1993-2006 that left more than 300 000 people dead.A peace deal, signed in the Tanzanian city of Arusha in 2000, paved the way to ending the fighting and included a provision that no leader could serve more than two five-year terms.Nkurunziza's third term circumvented that clause and the proposed constitutional amendments will abolish it, increasing terms to seven years and allowing Nkurunziza to stand again in 2020.Other reforms weaken constitutional constraints over the feared national intelligence agency and allow the revision of ethnic quotas seen as crucial to peace after the war.The new constitution also gets rid of one of two vice-presidents and shifts powers from the government to the president.Critics say the amendments threaten the Arusha peace agreement, with exiled opposition group CNARED calling it the "death knell" of the pact.The government has accused dissidents, and neighbouring countries, of planning to undermine the referendum and has deployed military units to areas bordering Rwanda to the north and Congo to the west.Earlier this month Burundi's press regulator suspended broadcasts by the BBC and Voice of America (VOA) and warned other radio stations, including Radio France International (RFI), against spreading "tendentious and misleading" information.