The opposition groups said they would boycott both parliamentary elections planned for Monday, and a presidential vote set for July 15.
Civil society groups backed the move in a joint statement calling on voters to skip the "sham elections" and urging the international community "not to recognise the validity" of the polls.
"Thousands of Burundians have fled the country, a thousand peaceful demonstrators were arrested, tortured, and are currently languishing in jail," the statement said.
"All the opposition have unanimously decided to boycott the elections," said Charles Nditije, a key opposition leader, after a letter signed by representatives of all the country's opposition groups was handed to the election commission.
Former colonial power Belgium said today it would not recognise the results of the elections, saying it was "impossible" that polls could be held in an "acceptable manner."
UN Secretary General Ban Ki-moon called today for the polls to be delayed, although similar such calls in the past have been dismissed by the government.