The order to marry follows the launch of a campaign this month by President Pierre Nkurunziza "to moralise society".
Unmarried couples have until the end of the year to legalise their relationships, the Burundi government said on Friday, as part of an effort to reform morals in the country.
The order follows the launch of a campaign this month by President Pierre Nkurunziza "to moralise society" in the tiny nation which for two years has been in the grip of sometimes violent political upheaval.
Interior ministry spokesman Terence Ntahiraja said the country was facing a population explosion which he blamed on "illegal marriages", polygamy, bigamy and "hundreds of schoolgirls getting pregnant".
He said church and state-sanctioned weddings were the solution and were a patriotic duty.
Nkurunziza said Burundians should show their love for each other -- and their country -- by getting married.
The government has since been pressuring unwed couples across the country to tie the knot.
Unwed couples on special list
The governor of the southeastern province of Rutana has ordered that "persons living in common-law unions" should be put on a special list by 22 June, while the governor of the northwestern Bubanza province has demanded unspecified "sanctions" against aisle-dodgers.
Pierre, a 27-year-old farmer living with his partner in Ngozi, in the north, said local officials had threatened him with a 50,000 Burundian franc ($25) fine and said any child born out of wedlock would not be eligible for free education and medical costs.
Pierre said he had not married because he could not afford the bride price demanded by his girlfriend's family.
"She told me she was pregnant. As I am poor, we decided to come together to raise our child," he said. "We thought we would legalise our union as soon as we could afford it."
That was five years ago and the couple is now onto their third child.
Mass weddings being organised
To enact the president's orders, officials have begun organising mass weddings, something one civil society activist opposed as "a violation of human rights because the state has no right to attack two adults who have decided to live together without being married."
The activist said the forced marriages were part of a "religious crusade" led by Nkurunziza and his wife, both fervent, born-again evangelical Christians.
Spokesman Ntahiraja dismissed such arguments saying the government's campaign was within the law.
"We want Burundians to understand that everyone is responsible for his life, we want order in this country," he said.
"All this is done within the framework of the patriotic training programme," he added, referring to an initiative launched by Nkurunziza in August 2013 to reinforce "positive traditional values."
Since 2015, when Nkurunziza ran for a controversial third term, at least 500 people have been killed in political violence while around 400,000 have fled Burundi for refugee camps in neighbouring countries.