Burundi is voting Thursday in a referendum proposing constitutional changes that could extend the president's rule until 2034.
Five million people are registered to vote in the referendum amid fears of violence because of President Pierre Nkurunziza's efforts to prolong his stay in power despite widespread opposition.
This east African country descended into violence in 2015 when Nkurunziza sought a disputed third term, sparking angry street protests that the authorities violently put down.
In Bujumbura, the capital, there are long lines of voters and the security forces have been deployed across the city.
Polls close at 6 p.m. local time. It is not clear when final results will be announced.
Nkurunziza, who forcefully campaigned in favor of amending the constitution, voted in his home province of Ngonzi.
The proposed changes include extending the length of the president's term from five years to seven. If the results of the referendum go his way, Nkurunziza could get an additional 14 years in power when his current term expires in 2020.
Nkurunziza's opponents want him to go, saying he has ruled longer than Burundi's constitution allows.
Tensions rose after unidentified attackers armed with machetes and guns carried out a massacre last week in Burundi's rural northwest near Congo, killing 26 people, many of them children. The government blamed a "terrorist group."
The 54-year-old Nkurunziza, a former rebel leader, rose to power in 2005 following the peace deal ending a civil war in which some 300,000 people died. He was re-elected unopposed in 2010 after the opposition boycotted the vote.
Protests erupted in April 2015 after Nkurunziza said he was eligible for a third term because lawmakers, not the general population, had chosen him for his first term. Critics called a third term unconstitutional, as the deal ending the civil war says the president can be re-elected only once.
The referendum campaigns were marred by allegations of violence and hate speech, including threats of drowning and castration.
Rights groups cited abuses targeting opponents of the proposed constitutional changes, including alleged beatings and killings.
The run-up to the referendum was "tainted by violence and increasing repression of dissent," according to Amnesty International.
"The reports of arrests, beatings and intimidation of real and perceived opponents of the constitutional amendments suggest the human rights situation in Burundi is only getting worse," the right's group's Rachel Nicholson said in a statement ahead of the vote.
Burundi's government strongly denies allegations it targets its own people, saying the charges are malicious propaganda spread by exiles. Authorities have imposed temporary broadcasting bans on the BBC and Voice of America, citing alleged violations.
The international community, however, has long expressed alarm. An estimated 1,200 people have been killed since early 2015, and International Criminal Court judges last year authorized an investigation into allegations of state-sponsored crimes.
Nkurunziza is one of a growing number of African leaders who are changing their countries' constitutions or using other means to prolong their stay in power. In neighboring Uganda President Yoweri Museveni, in power since 1986, could now rule until at least 2031 after lawmakers last year voted to remove the presidential age limit. In Congo President Joseph Kabila has overstayed his term by delaying elections.
Some in Burundi's opposition say they have little choice but to fight back. That could mean rebellion and even more bloodshed.
Hussein Radjabu is an opposition figure who once was a Nkurunziza ally and who was jailed for treason but escaped and fled the country. Speaking on the phone from outside Burundi, Radjabu told The Associated Press this week that the "only available option now is to use guns and we are determined to use all means to realize our cause."