BANGKOK — Hundreds of thousands of people were blocked from voting on Sunday as antigovernment demonstrators obstructed polling places in Bangkok and southern Thailand in a campaign to suspend democracy and replace Parliament with an unelected “people’s council.”

In a day of sometimes tense confrontations between protesters and would-be voters, one protest leader was shot dead by an unknown assailant and 11 people were wounded, according to Bangkok’s emergency services. Suthin Tharatin, a leader of one faction of the protesters, was shot as he tried to block a polling place on the outskirts of Bangkok, heightening fears of more widespread violence.

The protest movement is battling to purge the country of Prime Minister Yingluck Shinawatra and her influential family, and critics of the effort called Sunday’s shutdown of polling places a major blow to democracy in Thailand and a possible portent of further moves to seize power from the government.

More than two million people out of a total electorate of about 48 million were registered for Sunday’s advance voting, which was being held for those unable to vote in the Feb. 2 general elections. Ms. Yingluck’s party is considered almost certain to win the elections, which the main opposition party is boycotting.