The flags are being raised today ahead of the law taking effect tomorrow.
It marks a profound shift in attitudes in a country that little more than a decade ago had a law on the books banning the "promotion" of homosexuality.
Among the first to wed will be Sean Adl-Tabatabai and Sinclair Treadway. They will marry just after midnight in front of 100 guests at London's Camden Town Hall, before emerging to the strains of "I Got You, Babe" by Sonny and Cher.