More than half a million Hong Kongers have voted in an unofficial referendum on democratic reform in the specially administered Chinese city that Beijing has blasted as illegal.

About 26,500 people cast ballots at 15 polling stations around the city on Sunday. About 636,000 ballots have been cast since voting started Friday, most of them online or through a smartphone app.

Tensions have soared in Hong Kong over how much say residents of the former British colony can have in choosing their next leader, who's currently hand-picked by a 1,200-member committee of mostly pro-Beijing elites.

Beijing has pledged to allow Hong Kongers to choose their own leader, known as the chief executive, starting in 2017. But the central government has balked at letting residents nominate their own candidates.