An opposition coalition in Burundi has said it will not accept the results of a referendum on constitutional reforms that could extend President Pierre Nkurunziza's time in office, calling the process a "parody" and undemocratic.

In comments made on Saturday to The Associated Press, Agathon Rwasa, head of the Amizero y'Abarundi bloc, alleged that government forces had arrested perceived opponents ahead of Thursday's ballot and threatened to assassinate those who voted against amending the constitution.

The referendum asked voters to decide "yes" or "no" on proposed constitutional amendments, including whether the current five-year presidential terms will be increased to seven years. No change to the limit of two terms in office is proposed.

If approved, Nkurunziza, whose current term expires in two years, could run for two more terms and be in power until 2034.

"We will not accept the outcome of this referendum because it is a fantasy," said Rwasa, one of the few opposition leaders still in Burundi, a country plagued by a political and security crisis since 2015.

"Some of our members have been kidnapped, others beaten ... while some people were forced to vote 'yes' and during counting our representatives were expelled."

Rwasa called on Burundi's Electoral Commission to redo the ballot in a free and fair manner.

More than five million people registered to vote in the referendum, according to the commission, with official results expected to be announced imminently. The changes will be adopted if more than 50 percent of voters selected "yes".

Results published on Friday by a group of 15 public and private radio stations showed a majority "yes" vote in at least 14 of Burundi's 18 provinces.

'Widespread abuse, fear and pressure'

Before the vote, human rights groups said the campaign period had been marked by intimidation and abuse. 

And after the ballot, Human Rights Watch (HRW) said at least 15 people opposing the referendum had been killed, while six others were raped and eight abducted.

The "full scale of abuse" is likely "significantly higher", HRW said on Friday, with "many victims and witnesses unwilling or unable to report abuses".

Ida Sawyer, HRW's central Africa director, said the vote had taken place amid "widespread abuse, fear and pressure - a climate that is clearly not conducive to free choice".

But Willy Nyamitwe, Nkurunziza's presidential spokesman, praised the "peace" in which the ballot was held.

Constitutional controversy

The referendum comes three years after Nkurunziza won a controversial third term as president during a 2015 election, which was boycotted by the opposition.

Nkurunziza first came to power in 2005, five years after the signing of a nationwide peace deal known as the Arusha Accords, which paved the way for the ending of a 12-year-long civil war that saw more than 300,000 people killed.

According to a provision in the peace deal, no leader could serve more than two five-year terms.

But in early 2015, Nkurunziza claimed it was legal for him to run in the elections because for his first term he had been appointed to the presidency by parliament - as opposed to being voted in.

A political crisis ensued, with the opposition calling Nkurunziza's bid unconstitutional. An attempted coup was quashed, while a crackdown on anti-government protests resulted in the killing of at least 1,200 people.

More than 400,000 others, including opposition leaders, fled the country. 

The International Criminal Court has said it is investigating alleged state-sponsored crimes against humanity in the country. Last year, Burundi became the first country to leave the Hague-based court.

As well as allowing Nkurunziza to prolong his rule, Thursday's proposed changes to Burundi's constitution would also allow the revision of ethnic quotas currently protected under the accords.

The new document would also scrap one of the country's two constitutionally enshrined vice presidents and shift a number of powers away from the government to the president.