Leaders of the opposition groups at a joint press conference in Minsk on September 21 denounced the elections as a farce marked by the detention of political activists.
Anatol Lyabedzka, chairman of the United Civic Party, said the September 23 vote -- like previous elections in Belarus -- would be a sham and could as well be cancelled.
"Either we bring real elections of the president and [parliamentary] deputies back to Belarus or it is better to cancel the imitation [of democracy] we have witnessed," Lyabedzka said.
Vital Rymasheuski, co-chairman of Belarusian Christian Democracy, said there can be no free or fair elections while opponents of President Alyaksandr Lukashenka remain in jail as political prisoners.
"The isolation of the main opponents of the regime, who are thus prevented from participation in the election process, is alone enough to consider this election illegal and undemocratic," Rymasheuski said.
Opposition leaders said an election boycott would be a vote of no-confidence in Lukashenka's regime.
The United Civic Party and the Belarusian Popular Front, the two main opposition parties, announced last weekend they had withdrawn their candidates from the election.
Lyabedzka of the United Civic Party has urged voters to "go fishing, or visit your parents" instead of casting a vote.
The vote is expected to elect a rubberstamp parliament, with most powers remaining in the hands of Lukashenka -- who has ruled Belarus since 1994, continuing to crack down on dissent and independent media.
Rights groups say some 15 political prisoners remain jailed out of the dozens arrested during a crackdown on peaceful protests after the 2010 presidential election, when -- according to official results -- Lukashenka won a fourth term.
Amnesty International said the run-up to the voting had been marked by arrests and detention of opposition members.
The head of Belarus's Central Election Commission on September 17 criticized an announcement by the United Civic Union and the Belarusian Popular Front one day earlier to withdraw their candidates from the legislative elections.
Commission Secretary Mikalay Lozovik said the decision showed "disrespect for voters" and represented a "travesty of law."