Wedding bells were to ring out in Berlin, Hamburg, Hanover and other German cities where local authorities have exceptionally opened their doors on a Sunday, allowing weddings on the day the law comes into effect.
Bodo Mende, 60, and partner Karl Kreile, 59, exchanged their vows at a desk decked out with white flowers and rainbow flags.
Then, they turned to offer a shot of their first embrace as a married couple to the throng of photographers and TV crews from around the world who packed the south Berlin registry office alongside their friends and family.
Mende and Kreile, longtime gay marriage campaigners who have been together since 1979, wanted to tie the knot as soon as possible -- after being among the first to enter a civil partnership back in 2002.
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"This is a day we can really call historic," Angelika Schoettler, mayor of the capital's Tempelhof-Schoeneberg district, told the pair.
"I hope a great many couples will seal a bond for life in this room, or others like it."
Germany becomes the 15th European nation to legalise gay marriage.
The Netherlands led the way in 2000, followed piecemeal by other European countries including Spain, Sweden, Britain and France.
Same-sex relationships have become so normalised that polls show around 75 per cent of Germans are in favour of gay marriage.
By extending existing law to same-sex pairs, Germany's gay couples automatically gain the same tax advantages and adoption rights as heterosexual families.
But progress was slow and since 2001 gay couples have had to make do with a civil partnership law, broadened over the years to remove more and more gaps between gay and straight couples' rights.
The final breakthrough came quite suddenly in the end, sparked by Merkel's unexpected announcement in June that she would allow her conservative MPs to vote their conscience on the issue.
Merkel said her thinking changed after a "memorable experience" when she met a lesbian couple who lovingly care for eight foster children in her Baltic coast constituency.
The chancellor herself voted against the bill, arguing that the German constitution still defines marriage as "the union of a man and a woman".
"I still think it was indecent to delay for so many years, and the fact that she voted no," lawmaker Johannes Kahrs, gay and lesbian affairs commissioner for the Social Democratic Party, told AFP.
June was a memorable month for gay rights in Germany, as MPs also voted to quash the convictions of thousands of men convicted under a Nazi-era law against same-sex relationships which had remained on the statute book until 1994.
And the constitution must still be amended to fully protect against discrimination over gender or sexual orientation, Kahrs insisted.
"These are all things that we'll tackle bit by bit," he said.
"The important thing is that we've pushed through the opening of marriage, and that's the signal everyone needed.