Burundi's ruling party won all 100 parliamentary seats in last week's election, according to provisional results.
Burundian officials and youth groups "intimidated, harassed and threatened" voters to win every seat in parliamentary elections, Human Rights Watch said on Thursday.
The desperately poor Great Lakes nation held elections on June 5 in which the incumbent CNDD-FDD party swept 96 percent of the vote and all 100 seats in parliament, according to provisional results released on Wednesday.
The result was quickly condemned by opposition candidates, who alleged the vote was "rigged" through forced voting and restricted access.
HRW said in a statement that it had interviewed activists, journalists, citizens and a member of the ruling party's youth league.
They "spoke of intimidation and irregularities in both the lead-up to the election and during the voting", the rights group said.
"Ruling party officials and youths intimidated, harassed and threatened the population and censored media coverage to secure a landslide victory," it added.
HRW senior Great Lakes researcher Clementine de Montjoye characterised the election as "devoid of genuine political competition", noting the incumbent party "took no chances".
"Burundi's democracy has been hollowed out, with a ruling party unaccountable to its people and unwilling to tolerate dissent, even as economic desperation grows," de Montjoye said.
"Without credible opposition, this election only further entrenches authoritarian rule and pushes Burundians further into a deeply rooted governance crisis."
Members of the incumbent party's youth group -- the Imbonerakure -- were stationed in front of polling stations "telling people to vote for the ruling party", one voter told HRW, describing how all poll workers were party members.
"The head of the polling station himself told me to vote for the ruling party," they said.
In some places, the number of votes cast reportedly exceeded the number of registered voters, HRW said.
President Evariste Ndayishimiye took power in June 2020 after the death of his predecessor Pierre Nkurunziza, who ruled Burundi with an iron fist for 15 years.
Since taking office, he has swung between gestures of openness and a firm grip on power, with rights abuses denounced by NGOs and the UN.
A reporter told HRW that election officials told the media "not to talk about irregularities".
But journalists, who requested anonymity for security reasons, previously told AFP of significant issues.
Burundians face a dire economic outlook with severe petrol shortages that have crippled its ability to function, and the World Bank estimating inflation running at 40 percent.
The issues have been compounded by global aid cuts and an influx of refugees from the ongoing conflict in neighbouring Democratic Republic of Congo.