A military government seized power in November following a disputed election in which former President Umaro Sissoco Embaló and opposition candidate Fernando Dias both claimed victory.
If passed, the new constitution by the self-styled National Transitional Council will grant more powers to the president and reduce parliamentary representation, among other issues.
The authorities argue that a parliament trimmed from 102 to 65 seats would allow for easier and more robust debates, while new presidential powers would underline the higher authority of the president over the prime minister, which they claim is a source of political instability in the country. Opposition groups reject the claim.
"In a community, there is only one leader. No community or village can have two leaders. We revised the constitution to avoid constant conflict between the president of the republic and the prime minister. We clarified the law and helped the public understand that the president of the republic is the national symbol. It is the president who stands for election in presidential polls," General Tomás Djassi, the head of the armed forces, said.
But the opposition has called on voters to boycott the referendum, saying it is a path for the head of the military government to amass power and calling it a "source of political instability at all levels".
"This constitution significantly weakens the prime minister and the government, subordinates the National People's Assembly and political parties, and devalues - or even ignores the popular will expressed at the polls, carrying all the attendant risks of political and social division and tension," Muniro Conté, spokesperson for the African Party for the Independence of Guinea and Cape Verde, told The Associated Press.
Last year's coup in Guinea-Bissau worsened West Africa's democratic crisis, adding it to a growing number of countries in the region under military rule.
Leaders on the continent branded it a "staged" takeover after the military allowed the former president to host a press conference to announce he was being deposed and allowed to leave the country in seemingly favourable circumstances. Sceptics say Embaló had given up the seat to the military after seeing he was losing the election.
Since it gained independence from Portugal in 1973, the only Portuguese-speaking West African country has been rocked by several coups and attempted coups. Before last year's coup, a failed coup was reported only a month before.
The tiny country of around 2.2 million people has the highest proportion of natural wealth per capita, according to the World Bank, but more than half of the country lives in poverty.
