With 29 percent of the state's counties reporting, the measure to insert the ban on same-sex marriage in the state Constitution was passing 61.33 percent to 38.67 percent against. The Charlotte Observer said that trend would carry the amendment to passage.
Election officials said the turnout was the largest for a primary election in decades, the newspaper said.
"This was an issue of standing on the principle of God's word that marriage is between one man and one woman, and I believe that message has gotten across," the Rev. Mark Harris, the pastor of First Baptist Church of Charlotte who helped lead the campaign for passage, said.
The Rev. Robin Tanner, the pastor of Piedmont Unitarian Universalist Church in Charlotte, who was on the losing side, indicated in a statement he had hope of a reversal down the road.
"Hope lives on in this place we all call home," he said. "Hope is our promised companion, and equality for all our promised land."
The amendment, known as Amendment 1, stipulates "marriage between one man and one woman is the only domestic legal union that shall be valid or recognized in this state."
The amendment, which takes effect Jan. 1, not only bans recognition of same-sex marriage, but also civil unions and domestic partnerships of any kind.
"The people of the state recognize the importance of protecting marriage between a man and a woman," Tami Fitzgerald, another pro-amendment leader told Fox News Channel prior to the polls closing.
"We didn't want civil union to be a marriage clone," Republican state Sen. Dan Soucek, one of the amendment's primary sponsors, told Fox.
"We wanted that institution to be protected, not just the word."
About 12 percent of North Carolina domestic partnerships are between same-sex couples, 2010 U.S. Census data indicate.
Evangelist Billy Graham last week urged voters to support Amendment 1.
"At 93, I never thought we would have to debate the definition of marriage," Graham, a North Carolinian, said in a rare statement. "The Bible is clear -- God's definition of marriage is between a man and a woman.
"I want to urge my fellow North Carolinians to vote for the marriage amendment," his statement said.
The Coalition to Protect North Carolina Families, which opposed Amendment 1, raised $2.2 million. Vote For Marriage NC, which supported the marriage-defining amendment, raised $1.1 million.
A poll conducted by Civitas Institute, a conservative think tank, had projected the measure would win by 16 percentage points.
A poll by Public Policy Polling, which often conducts polls for Democratic clients, had indicated the measure would win by 14 points.
North Carolina law already bans same-sex marriage, but the state had not changed its Constitution to ban it.
Ten other Southern states have constitutional bans on same-sex marriage and, in some cases, on civil unions as well.