At least two large Chinese state-owned enterprises in Hong Kong are instructing staffers how to vote in Sunday's legislative election as Beijing seeks to thwart democratic candidates.

The election is Hong Kong's most contentious since the 1997 handover to China, marking the most significant city-wide poll since the 2014 "Occupy Central" protests, when tens of thousands took to the streets to demand full democracy.

Pro-Beijing politicians are seeking to win enough of the 70 seats in the Legislative Council to break the one-third veto-wielding bloc held by pro-democratic parties in the freewheeling global financial hub of 7 million people.