Crowds gathered to greet Limam Chafi on his return from exile in Qatar on Sunday.
Chaffi, a sharp critic of former president Mohamed Ould Abdel Aziz fled Mauritania after charges were pressed against him.
Chaffi's return points to changing political dynamics in Nouakchott.
"We think it is a new page that is opening up in which all Mauritanians will have their place'', said Chaffi as he spoke to reporters at his home.
Under president Mohamed Ould Ghezouani, Mauritania has freed or dropped charges against critics of the former regime and allowed prominent exiled figures to return.
Former army general Ghezouani, became president in 2019, succeeding Abdel Aziz in the northwest African country's first peaceful transfer of power.