Opposition groups said Saturday they would not recognise the results.
The result of a referendum on constitutional reforms that could leave President Pierre Nkurunziza in power until 2034 will be declared Monday, the electoral commission (Ceni) said Sunday.
"Announcement of provisional referendum results Monday May 21 ... start of ceremonies 16.00 (1400 GMT) precisely," said Ceni head Pierre-Claver Ndayicariye in a message to political parties seen by AFP.
The constitutional court will thereafter have to give its official seal to the outcome of a vote which looks assured but which the opposition has decried as an undemocratic foregone conclusion.
A Ceni source said all parties were invited along with national observers and diplomats to the Monday gathering.
Opposition groups said Saturday they would not recognise the result of Thursday's vote.
They expressed amazement Sunday that the Ceni had given no indication of the final result, albeit it had said Friday that provisional returns from all but one of the country's 18 provinces showed majority support for constitutional reforms.
Ceni said it was awaiting final returns before making a pronouncement.
"We know the Ceni and the government have used this period (since polling day) to massage the figures," said one opposition member who requested anonymity and who said the results would be made to fit the official narrative.
Main opposition leader Agathon Rwasa said Saturday that "the electoral process has been neither free nor transparent, nor independent and still less democratic," adding he rejected the "fantasist results" which would in due course emerge.
Nkurunziza, first elected by parliament in 2005, won reelection and then a third, much contested, poll in 2015.
But his announcement he was standing again that year despite being constitutionally limited to two terms sparked an attempted coup and a crackdown which cost at least 1,200 lives and left more than 400,000 homeless.
Should he manage to have his proposed reforms passed he will be able to target two more, this time seven-year, terms from 2020.
The proposed reforms also weaken constitutional constraints over Burundi's feared national intelligence agency and allow the revision of ethnic quotas seen as crucial to peace.
The new constitution would also scrap one of two vice-presidents and shift powers from the government to the president.