The results of Thursday's referendum in Burundi which could allow President Pierre Nkurunziza to stay in office for 17 more years are due later today.
They are expected to show a big majority in favour of the constitutional change after voters were asked to accept or reject a proposal that would allow the president to stand for two seven-year terms.
However, reports from polling stations say some people were being forced to vote to avoid being beaten or arrested.
Foreign journalists were mostly denied access to the country during the vote.
Opposition voices have been largely silenced.
Human Rights Watch said Friday that at least 15 people were killed and six were raped during a referendum campaign.
Security forces and their allies the Imbonerakure youth militia created a climate of fear and intimidation ahead of the vote and opponents were targeted, said the rights group in a report.
Suspected opponents were “killed, raped, abducted, beaten, and intimidated,” Human Rights Watch said in a statement, adding it had documented at least 15 killings, six rapes and eight abductions.
“Burundi’s referendum took place amid widespread abuse, fear and pressure, a climate that is clearly not conducive to free choice,” said Ida Sawyer, HRW’s central Africa director.
President Nkurunziza is a former rebel leader who took power in 2005 at the end of a civil war in which around 300,000 people were killed. His current term ends in 2020.
He was due to step down in 2015 but early that year he announced he was seeking a third term, triggering deadly clashes with his opponents and a political and security crisis that has gripped the impoverished country of 10 million since.
Half a million people have been displaced, mostly as refugees in Rwanda, Tanzania and Uganda.