The government-run Agency Boliviana de Informatics (API) reports that Ars has been sworn in by Bolivian legislature Speaker David Chokhuanga.
The event, held in La Paz, was attended by regional and world leaders, including King Felipe VI of Spain, as well as leaders from Paraguay, Argentina and Colombia, CNN Espanol reported.
Senior officials from Chile, Iran and Venezuela’s Nicolas Maduro were also in attendance.
Morales did not attend the ceremony, but will return to Bolivia on Monday, the API said. He is then expected to head to his rural stronghold of Sapporo.
Ars will be Bolivia’s 67th president, according to the API. His inauguration follows his victory in Bolivia’s October election.
The former economy minister, who is considered the architect of Bolivia’s rapid growth under Morales, will come to power trying to heal the political crisis and divisions of the corona virus epidemic. He is a member of the Bolivian Movement Socialism (Mass) Party.
Tensions are high in the country over changes made by the socialist-controlled Congress to reduce the majority needed to pass new laws since Morales was deported to Argentina, where he has lived since he left Bolivia last year.
Morales, who led the country for nearly 14 years as the first indigenous president, provoked widespread setbacks last year when he faced an unprecedented fourth time limit and national referendum.
After claiming victory in that election, Morales was forced to resign after angry protests, and was eventually backed by police and the military. After a while he left the country.
Ars, who was elected and a close ally by Morales, has said the former president will not play any role in his government.
As Minister of Economy, Ars promoted the nationalization of strategic sectors such as hydrocarbons and energy, and pushed domestic production with public investments and subsidies, which helped Bolivia grow at a very rapid rate in Latin America.