One of the first mixed-sex couples to become civil partners hailed it as a "unique, special and personal moment".
Rebecca Steinfeld and Charles Keidan, who won a legal battle for the right to heterosexual civil partnerships, celebrated at Kensington and Chelsea Register Office in west London.
Previously, the law only allowed same-sex couples to be civil partners.
About 84,000 mixed-sex couples could form civil partnerships next year, the government says.
Introduced for same-sex couples in 2005, civil partnerships offer almost identical rights as marriage, including property, inheritance and tax entitlements.
After Ms Steinfeld and Mr Keidan won their legal bid at the Supreme Court in 2018 for the right to have a civil partnership instead of a marriage, the rules were changed to make them available to everyone in England and Wales.
The Civil Partnership (Scotland) Bill is currently making its way through the Scottish Parliament to make mixed-sex partnerships legal north of the border.
And in Northern Ireland, a law which requires the government to legalise same-sex marriage next month also requires it to extend civil partnerships to opposite-sex couples.
Speaking on the steps of the register office, Ms Steinfeld said their "personal wish" to form a civil partnership came from a "desire to formalise our relationship in a more modern way, with a focus on equality, and mutual respect".
She said: "So today is a unique, special and personal moment for us, a moment that we've been able to affirm our love and commitment to one another in the company of our beautiful children, Eden and Ariel, and close friends."
Ms Steinfeld said it creates "new, modern possibilities" for thousands of people to express their love and commitment and ends "the unrivalled position of marriage".
She called for "deeper discussions" on giving legal recognition to other kinds of caring relationships, including those between friends, siblings and co-parents.
Mr Keidan said they succeeded in their legal battle "against all odds" but added that their mental health has suffered under the strain.
Five years after being refused permission to give notice of a heterosexual civil partnership, Rebecca Steinfeld and Charles Keidan will finally become civil partners today.
Their conscientious objection to marriage and what they saw as its patriarchal associations led to a lengthy legal battle culminating in a unanimous Supreme Court ruling last year that the law was discriminatory and breached their right to a family and private life.
The government changed the law, opening such a union to the majority of the UK's 3.3 million co-habiting heterosexual couples.
Many believe they are already protected by so-called "common law marriages", but these do not exist.
As a result, they do not enjoy the same property, inheritance and tax entitlements as married couples and civil partners.
The government estimates as many as 84,000 mixed sex couples could become civil partners this year, giving them greater rights and protections within their relationships, without having to get married.
Cathy Brown and John Grisswell, from Wirksworth, Derbyshire, are another mixed-sex couple to go through the civil partnership ceremony on the day it became possible.
They had been married to other people before and wanted an alternative option.
Ms Brown said she and Mr Grisswell felt "strongly" that "repeating those vows and promises, knowing they hadn't worked the first time, wasn't the route we wanted to go down".
Mary Ann Lund and Gareth Wood, from Market Harborough, Leicestershire, also went through a civil partnership ceremony on Tuesday.
"It's more about the equality of a partnership rather than a marriage," Dr Lund said.
"That's something important to us, that we feel there is a kind of historical, patriarchal baggage in marriage and it's not particularly something that's for us."
Another couple, Julie Thorpe, 61, and Keith Lomax, 70, said they were looking forward to being among the first mixed-sex people to officially enter a civil partnership - but it would not change their relationship "one jot".
The couple from near Hebden Bridge, West Yorkshire, have been living together for most of their 37-year relationship and have three children.
They will have a civil partnership ceremony at a register office in Halifax.
Ms Thorpe said: "It won't change our relationship one jot. It will not make any difference to how we behave towards each other when we get up the next day.
"We have had a very successful relationship for 37 years and a bit of paper is not going to make any difference to that whatsoever. It does give us some legal protection within that relationship."
Mr Lomax, a human rights lawyer, added: "It is a mutual celebration of all of those and also of the people who actually brought the case to court and changed the law in the first place, because that was a very brave and bold thing to do at considerable financial risk."