Fashion entrepreneur Mary Portas surprised her civil partner as they became one of the first couples in the UK to convert their civil partnership to a same-sex marriage.
The conversion took place at Westminster City Hall in London at 12.01am today, one minute after the law came into effect.
Westminster superintendent registrar Alison Cathcart presided over the conversion of Ms Portas and fashion journalist Melanie Rickey's civil partnership, and said she saw it as the "final step in equality for gay marriage" and Westminster City Hall was "very proud to play our part in it".
Ms Portas described the occasion as "absolutely magical" and said she had been planning it for about a month.
She said: "We have three children together and there is such a deep rooted commitment to being married, it just felt right.
"Our last ceremony was very different, with a much wider extension of friends and family."
Ms Rickey said: "I thought we were out looking for art galleries but when we started driving up and down the same street a couple of times I started getting a bit suspicious.
"When we turned up outside Westminster City Hall I guessed what was going on.
"I couldn't think of a better surprise."
Deputy Prime Minister Nick Clegg praised the change to the Marriage Act as further progress for LGBT (lesbian, gay, bisexual and transgender) equality in the UK, and said: "I am proud that couples can finally convert their civil partnerships to marriage."
He said today was another celebration for LGBT rights in this country, but it was also a reminder that there are people across the world living under much less fortunate conditions, facing discrimination and persecution on a daily basis.
He added: "So as we raise a glass to those making their vows, we send a message of solidarity to LGBT people everywhere."
Ten same-sex couples also made history in Brighton and Hove as they simultaneously converted their civil unions.
Ten registrars took the couples through the administrative process simultaneously in separate offices at Brighton Town Hall at 10am and all the couples were given a commemorative certificate to mark their involvement in the day's celebration.
Ian Scott, 70, and Ron Williams, 82, from Hove, whose civil partnership took place in January 2006, were among the couples taking part today.
When they met and fell in love in 1962 homosexuality was still illegal and the couple said they never thought they would see the day when they could be legally married.
Mr Williams said: "It's a very good feeling and we are very happy that we have been able to do this. We have waited 52 years."
John Azzopardi, 39, and Aaron Burns, 38, from Hove, also took part in the conversion at 10am.
Mr Azzopardi said: "It's a really important step for same-sex couples as it is about our marriage being equal under the law. It is important for all same-sex couples to have the same equality if they want it."
Linda Holm, superintendent registrar, said about 150 couples had already booked to convert their civil partnerships into marriages and that following this morning's conversions, another six or seven would be taking place at the town hall.
She said: "The atmosphere has been great, really positive and exciting. For those couples that have been through the process this morning it has been life-changing. It's a great privilege to be part of that and to help them through the legal process."
The Marriage (Same Sex Couples) Act came into effect on March 29 but there was no legal way for couples in civil partnerships to be married until Parliament moved to amend the law.
Couples wishing to convert their civil partnerships will have the choice of a simple process at a register office or a two-stage process, where a registrar can complete the conversion at another venue meaning the couple's family and friends can attend.
The conversion can take place at premises which have been registered for the marriage of same-sex couples - such as hotels, stately homes and religious premises.
For the first year, all couples who formed their civil partnership before March 29 (when same-sex marriage was introduced) will be able to receive a £45 fee reduction.