Unmarried couples in the East African country of Burundi have until the end of the year to legalise their relationships through church or state registrations.
The government order comes after President Pierre Nkurunziza launched a campaign “to moralise society” after signing the new law in May.
A government spokesman said a crackdown on informal relationships was needed to combat a population explosion.
He said too many schoolgirls were getting pregnant and men were taking advantage of women by cohabiting with several simultaneously.
Burundi has been in crisis since 2015 when Mr Nkurunziza, a born-again Christian, announced he would run for a controversial third term.
The government insists a legal document recognising a marriage helps protect women and their children, especially when it comes to issues such as inheritance. However, others say the new marriage law infringes on people’s religious beliefs, customs and practices.