Nicolas Maduro assumed the role of acting president at the death of Hugo Chavez in March 2013, and was declared winner of the presidential elections held the following month.
He was named as vice-president in October 2010 by Mr Chavez, who subsequently named him as his preferred successor.
The opposition candidate in the presidential election, Henrique Capriles, said he would contest the results of the vote.
Mr Capriles lost by less than two percentage points, but said his team had a list of more than 3,000 irregularities and demanded a recount.
Mr Maduro called for calm and for the result to be respected.
Analysts of the Venezuelan political scene say the narrow margin of victory leaves Mr Maduro with reduced authority and the difficult task of maintaining unity in a ruling alliance that includes military officers, oil executives and slum leaders.