"This is the moment when Mauritanians will come of age," he said.
About 2,400 polling stations will open between 7:00am (0700 GMT) and 7:00pm (1900 GMT) and voting is to be observed by some 300 international poll watchers.
A record 19 candidates jostling for the country's top job have ended their two-week campaigns which passed off peacefully.
"The campaign went by in absolute calm and without the slightest of incidents," Vall said.
The vote is the final stage in a transition of the country to civilian democracy after the coup ended 20 years of dictatorship under Ould Taya.
The junta promised to return the country to democracy and step down after the 19-month transition and barred its members from standing for political office to guarantee transparency.
It has so far organised a constitutional referendum plus local government and legislative elections.
Power has never before changed hands through elections in the former French colony, whose history has been chequered by some dozen coups and military takeovers since independence in 1960.
If no single candidate garners 50 per cent of the votes cast, the election will go to a second round on March 25.