The only independent observer monitoring the polls in Venezuela said that Sunday’s vote for president did not meet international standards and was undemocratic, raising more questions about the legitimacy of the results.

The mission, led by the Carter Center, a pro-democracy organization, said late Tuesday that the election violated Venezuela’s own laws and the government’s failure to release a vote count was a “serious breach of electoral principles.’’

The group joined the United States and many other countries that have said Venezuela’s election was marred by irregularities. At least 16 people have been killed in protests that erupted after election officials declared the country’s autocratic leader, President Nicolás Maduro, the winner.

The condemnation by the Carter Center, which was the lone independent election monitor the government allowed into Venezuela ahead of the vote, came hours after opposition leaders announced updated election results showing Mr. Maduro received less than a third of votes cast.

Venezuelans went to the polls Sunday to choose between Mr. Maduro, who has been in power since 2013, and Edmundo González, a former diplomat who served as a stand-in for María Corina Machado, a more popular opposition leader who had been barred by the government from running.

Ms. Machado released data showing that with more than 81 percent of the machines counted, Mr. González received 67 percent of the vote, compared to 30 percent for Mr. Maduro. The opposition’s calculations came from voting machine tallies provided to election observers, she said.