Kafando, appointed in the wake of violent protests that brought down president Blaise Compaore and led to a brief army power grab, pledged he would not let the country become a "banana republic".
The one-time foreign minister vowed to "respect and defend the constitution, the transition charter and laws and do everything to guarantee justice for all Burkinabes" as he took the oath at a televised ceremony in the capital Ouagadougou.
Poised to formally take over Friday from an interim military ruler, the 72-year-old emphasised his "humility" as a leader aware that he temporarily held "power that belongs to the people".
The landlocked nation of 17 million people "could never become a banana republic," he added, declaring that it was the "first elementary duty of a citizen" to respect the constitution.
Mass unrest erupted in late October over Compaore's bid to change the constitution, which would have allowed him to extend his 27-year rule of the former French colony.
Kafando, who served as Burkina Faso's UN envoy from 1998-2011 and as foreign minister in 1982-83, was chosen to head the transition after tortuous negotiations between the military, political parties and civil society groups.
The African Union had warned that the deeply poor country would face sanctions unless it chose an interim leader, and it welcomed Kafando's appointment.
However, the decision to hand power to a man closely associated with the former regime raised suspicions for many.
"Compaore said for 27 years that he was not thirsty for power, yet he remained firmly in his job," said Amadou Sawadogo, a 37-year-old street seller.
Burkina Faso notably exports cotton and gold, but almost half the population lives on less than one dollar a day and many are subsistence farmers.
Kafando, who was chosen by a panel of 23 officials, will be charged with appointing a prime minister -- either a civilian or a military figure -- who will head a 25-member transitional government.
A civilian will also head a 90-seat parliament, known as the National Transitional Council.
