Haitians are voting in legislative elections that have been repeatedly delayed since 2011, in a test of stability for the impoverished nation.
Police have been deployed as voters choose two-thirds of the senate and the chamber of deputies.
A presidential poll is set for October.
President Michel Martelly, who is constitutionally barred from running again, has ruled by decree since January. The opposition accuse him of abusing his powers.
Polling stations across Haiti opened at 06:00 local time (10:00 GMT) and will close at 16:00.
Nearly six million eligible voters are choosing 119 deputies and 20 senators from more than 1,800 candidates registered from different political parties.
The run-off round will be held on 25 October – the same day as the presidential election.
Several people have been killed since campaigning for Sunday’s poll opened in July.
Haiti’s police are being helped by UN police and peacekeepers to ensure safety during the voting.
“These elections will be a major test for October,” said Pierre Esperance, who heads the National Human Rights Defence Network.
Haiti’s parliament was dissolved in January over its failure to hold elections, leaving Haiti without a functioning government.
Haiti is one of the poorest countries in the world, and is still struggling with the legacy of the devastating 2010 earthquake.