The rights organization reported at least 20 percent of the 2 million people registered for early voting were unable to cast their ballots during the weekend because of interference from opposition protesters.
"The protesters claim they are fighting corruption and seeking reforms, but this doesn't justify their use of force and intimidation to block voting," Brad Adams, Asia director for Human Rights Watch, said in a statement Tuesday. "Preventing people from casting ballots shows serious contempt for basic rights of voters and democratic principles."
Adams said Thai authorities need to ensure opposition supporters are able to express their opinions without the threat of violence.
Parts of Thailand are under a state of emergency ahead of elections Sunday.
Prime Minister Yingluck Shinawatra and her allies are expected to win the election. Opposition groups threatened to boycott the process.
At least 89 people died as a result of political violence in Thailand in 2010.