BOGOTA (Reuters) - Leftist Gustavo Petro, a former member of the M-19 guerrilla movement who has vowed profound social and economic change, will be Colombia's next president after beating construction magnate Rodolfo Hernandez in an election on Sunday.

       Petro, a current senator and previous mayor of Bogota, won 50.8% of votes, figures from the national registry office showed, while Hernandez trailed by 797,973 votes, tallying 46.9%.

       Some 20 million people cast ballots nationwide.

       (Reporting by Julia Symmes Cobb; Editing by Cynthia Osterman)