RELATED: Read the NJ Supreme Court's decision online
The ruling puts New Jersey on the cusp of becoming the 14th state - and the third most populous among them - to allow same-sex marriage. As of Monday, one-third of Americans will live in a place where gay marriage is legal.
"The state has advanced a number of arguments, but none of them overcome this reality: Same-sex couples who cannot marry are not treated equally under the law today," the court said in an opinion by Chief Justice Stuart Rabner. "The harm to them is real, not abstract or speculative."
A judge on a lower court had ruled last month that New Jersey must recognize same-sex marriage and set Monday as the date to allow weddings. Christie, a Republican who is considered a possible 2016 presidential candidate, appealed the decision and asked for the start date to be put on hold while the state appeals.
A spokesman for Christie said that he will comply with the ruling, though he doesn't like it.
"While the governor firmly believes that this determination should be made by all the people of the State of New Jersey, he has instructed the Department of Health to cooperate with all municipalities in effectuating the order," spokesman Michael Drewniak said in a statement.
Same-sex marriage is being debated elsewhere. Oregon has begun recognizing same-sex weddings performed out of state, and it is likely that voters will get a chance next year to repeal the state's constitutional ban on gay marriage. The Hawaii Legislature also soon could take up a bill to legalize same-sex unions, while a similar measure has passed the Illinois Senate but not the House. Lawsuits challenging gay marriage bans also are pending in several states, including Michigan, Pennsylvania and Virginia.
New Jersey's top court agreed last week to take up the appeal of the lower-court ruling. Oral arguments are expected Jan. 6 or 7.
In Friday's opinion, Rabner wrote that the state has not shown that it is likely to prevail in the case, though it did present some reasons not to marriage to move forward now.
"But when a party presents a clear case of unequal treatment, and asks the court to vindicate constitutionally protected rights, a court may not sidestep its obligation to rule for an indefinite amount of time," he wrote. "Under these circumstances, courts do not have the option to defer."
Rabner also rejected the state's argument that it was in the public interest not to allow marriages until the court has had more time to rule fully on the issue.
"We can find no public interest in depriving a group of New Jersey residents of their constitutional right to equal protection while the appeals process unfolds," he wrote.
For those opposed to gay marriage, denying the request to delay was troubling.
"In what universe does it make sense to let the question at hand be answered before it's asked or argued?" Len Deo, president of the New Jersey Family Policy Council, said in a letter Friday to members.
On Thursday, some communities started accepting applications for marriage licenses from same-sex couples so that they would pass the 72-hour waiting period by 12:01 a.m. Monday.
Several communities, including Asbury Park, Lambertville and Newark - where Cory Booker, who was elected Wednesday to the U.S. Senate, is mayor - are holding ceremonies for multiple couples then.
"It's a great day to be gay in New Jersey," said Amy Quinn, a member of the city council in Asbury Park who is planning to marry Heather Jensen, her partner of 10 years, on Monday.
At a rally Friday night in Montclair, about 150 people gathered, cheering and holding signs reading "Support love." Some opened bottles of champagne.
Among the couples at the rally were David Gibson and Rich Kiamco of Jersey City, who had their marriage license in hand so they could get married Monday. The couple previously got married in New York but wants to get married where they live, they said.
The court did not address the question of what would happen to the status of same-sex marriages entered into next week if it later decides that the state does not have to grant the marriages.
Whether gay couples should have the right to marry in New Jersey has been the subject of a battle in the state's courts and Legislature for a decade. There has been a flurry of movements in both venues since June, when the U.S. Supreme Court invalidated key parts of a federal law that prevented the federal government from recognizing same-sex unions.
Since then, gay rights advocates have asked New Jersey judges to force the state to recognize same-sex marriage, arguing that the current policy of civil unions but not marriage licenses amounts to denying them federal protections such as Social Security survivor benefits and the right to file tax returns jointly.
Since July, gay rights groups have also engaged in an intense campaign aimed at persuading lawmakers to override Christie's 2012 veto of a bill that would have allowed gay marriage. To get an override, the Legislature must act by Jan. 14.
Sheila Oliver, speaker of the state Assembly, issued a statement blaming Christie for not having gay marriage sooner in New Jersey.
"It's a shame it took this long to get to this point and that it took a court fight for same-sex couples to gain equal rights," she said. "New Jersey could have had marriage equality already if it wasn't for Gov. Christie, who has done everything he could to prevent this from happening, including wasting money and time continuing this court battle."
Mulvihill reported from Haddonfield, N.J. Contributed to this report were Associated Press writers Samantha Henry in Newark, Mark Sherman in Washington, David Crary in New York and photojournalist Joe Epstein in Montclair.