Voters in North Carolina have approved a constitutional amendment defining marriage solely as a union between one man and one woman, in a defeat for gay rights advocates.

It will become the 30th state in the union to enshrine a ban on same-sex marriage in its state constitution. Same-sex marriage has been illegal in NC for 16 years but can now only be legalised by another vote by the people.

With more than 97% of precincts reporting late on Tuesday, unofficial returns showed the amendment passing with 61% in favour and 39% against. The amendment declares that "marriage between one man and one woman is the only domestic legal union that shall be valid or recognised in this state".

North Carolina was one of several state elections to be closely monitored by a wider audience on Tuesday. In Indiana, Senator Richard Lugar lost his re-election bid in the state primary, his 36-year career ended by a victory for the Republican state treasurer, Richard Mourdock, who is backed by the Tea Party and the National Rifle Association.

In Wisconsin, the Milwaukee mayor, Tom Barrett, won the Democratic primary in the state's gubernatorial recall election, confirming him to go up against the Republican governor, Scott Walker, in June.

Tami Fitzgerald, chairwoman of Votes for Marriage NC, the main group behind the amendment, said: "We are not anti-gay, we are pro-marriage. The whole point is you don't rewrite the nature of God's design for marriage based on the demands of a group of adults."

Supporters celebrated the win with a tiered wedding cake at a party in the North Raleigh Hilton Hotel. They said the amendment was needed to stave off those trying to redefine marriage and prevent any future action by judges.

Gay and lesbian rights groups expressed their disappointment in the result but said the fight had brought them together.

President Barack Obama's campaign said in a statement on Tuesday he was "disappointed" with the state's constitutional amendment banning gay marriage, describing it as "divisive and discriminatory". Obama has not supported legalising gay marriage but has said that his views on it were "evolving".

Jeremy Kennedy, of the Coalition to Protect all NC Families, which has been fighting the amendment, said: "It is just a skirmish in a battle, a war that we will win."

In an emotional speech, he told supporters they had "left no stone unturned" in bringing people together to fight the amendment.

"Tonight we walk away proud with our heads held high and we will continue to fight this."

Kennedy and other opponents of the measure warned it could result in a host of problems for unmarried couples, including erosion of their health benefits and those of their children. They said it could affect domestic violence laws to protect women.

Evan Wolfson, founder and president of Freedom to Marry, said the vote provided a sharp contrast with the momentum for the freedom to marry elsewhere in the US and described it as "a painful reminder of what happens when a pre-emptive ballot measure is stampeded through before people have had enough time to take in real conversations about who gay families are and why marriage matters to them."

In February, Wolfson and other gay rights campaigners celebrated a victory in California after a federal appeals court struck down the state's ban on same sex marriage, ruling it unconstitutional.

The Reverend Jasmine Beach-Ferrara, executive director of the Campaign for Southern Equality, said: "We are severely disappointed that Amendment 1

has passed. But we also now know that a growing number of North Carolinians support equality for LGBT people."

Twenty-eight states have already passed a constitutional amendment defining marriage as solely between a man and a woman. The amendment in North Carolina goes beyond state law by preventing other forms of domestic unions from carrying legal status.

Legal experts have warned that the broad wording of the amendment could cause a host of problems for unmarried couples. Several North Carolina municipalities provide benefits to unmarried couples in domestic partnerships and lawyers have told the Guardian those rights could be lost if the amendment is passed.

They say it could disrupt protection orders for unmarried couples and impact victims of domestic violence. The term "domestic legal union" is not defined by North Carolina law.

Holning Lau, an associate professor of law at the University of North Carolina, who has written extensively on the implications of Amendment 1, said: "The language is very broad compared to other states. It is a common misconception that it would only affect same-sex marriage."

A report by Lau and others concluded it was "impossible to predict" how courts would resolve issues such as protection for victims of domestic violence, raised by the amendment's vague language.

It concluded that it would take years of expensive litigation to settle its meaning and "when the dust clears [all] unmarried couples would have fewer rights over their most important life decisions than they would have had otherwise".

Earlier on Tuesday, a senior official at the board of elections in the state capital said the election to decide on a state constitutional amendment was the "craziest in 13 years".

Gary Sims, the deputy director of Wake County board of elections in Raleigh, told the Guardian there were "some really angry people" on both sides of the highly charged debate.

Observers from the Republican party had sought to "challenge and confront" precinct officials from the board and were "clogging up the phone lines" back at Wake County headquarters. "This is the craziest election I've seen in 13 years," said Sims, at his office next to the courthouse.

"We've seen political party observers who are not precinct officials. They can ask to be observers. They want to challenge and confront and it's a problem for our precinct officials."

He said the reason for the confrontations varied, from calling up the board of elections to complain that there wasn't a chair for them to sit on to pushing officials to get people to show ID at the polls.

"They have been clogging up the phone lines and getting mad at us," said Sims. "People have to state their names and addresses and we check them. But they are challenging officials to make them show ID. They have an agenda, the ones that we've been getting trouble from."

When asked whether the trouble was coming from the groups for or against Amendment 1, Sims said: "Put it this way: we had zero Democratic party observers." He added: "I've probably said more than I should."

High-profile figures, including Barack Obama and Bill Clinton, who recorded telephone calls to voters, had weighed in on the amendment debate, urging voters to reject it. Opponents also held marches, put out television ads and gave speeches, including one by Jay Bakker, son of televangelists Jim Bakker and the late Tammy Faye Bakker.

Billy Graham, the evangelical preacher who at age 93 remains influential in the state where he has a stretch of road named after him, was featured in full-page newspaper ads supporting the amendment.

Massachusetts, Iowa, Vermont, New Hampshire, Connecticut, New York and the District of Columbia allow gay and lesbian nuptials. Maryland, New Jersey and Washington state passed laws this year approving same-sex marriage, but Governor Chris Christie vetoed New Jersey's law and opponents in Maryland and Washington are threatening ballot initiatives to overturn those state's laws.