New York - New Yorkers began voting on Tuesday in an election that is likely to see billionaire Michael Bloomberg win an unprecedented third four-year term as mayor.

Opinion polls ahead of the vote put the 67-year old Bloomberg 12 percentage points ahead of his Democratic challenger William Thompson, 56.

Bloomberg spent more than 100 million dollars on his campaign, against 6 million by Thompson.

Bloomberg's personal wealth is estimated at 16 billion dollars, fuelled by the namesake financial news service he founded from scratch decades ago after holding a job on Wall Street.

New York media have kept a close watch over each of Bloomberg's moves and rarely found fault. When he asked the City Council to change laws increasing mayoral term limits from two to three, there was a muted outcry that did not last long.

Bloomberg was first elected mayor in the wake of September 11, 2001 terrorist attacks that killed more than 2,700 people in the destroyed World Trade Center and inflicted tens of billions of dollars in damage to the city's infrastructure, Wall Street firms and jobs. (dpa)