BELGRADE, Serbia -- Long lines of voters formed outside polling stations across Serbia on Sunday for a parliamentary election that is seen as a showdown between the Balkan country's strongman leader Aleksandar Vucic and a diverse movement of university students who have led protests against him for almost two years.

Though Serbia's 6. 4 million voters get to choose from 18 parties hoping to enter the 250-member Parliament, the election has taken shape as a referendum between Vucic's right-wing Serbian Progressive Party and the youth-backed Students' List.

The main contenders are believed to be polling neck-and-neck, making Sunday's election the most uncertain in recent years.