French and British supporters of traditional marriage are braving miserable weather conditions to rally in London's Trafalgar Square.
The rally on Sunday afternoon was organised by French protesters living in the UK who are angry with their government back home over plans to redefine marriage.
Proposals were approved by the French National Assembly in February by a margin of 329 to 229 and are due to be debated in the Senate in early April.
Organisers of today's rally say that the debate around legalising gay marriage has been dominated by the "desires of adults" rather than the wellbeing of children.
British people opposed to similar plans here were invited to stand together in the capital to make their concerns heard.
The rally was timed to coincide with a demonstration in Paris today and a similar protest taking place in Washington DC on Tuesday.
London co-ordinator Damien Fournier Montgieux, criticised the way in which the governments in France and the UK have handled the legalisation of gay marriage.
"Almost out of nowhere we are suddenly faced with a huge challenge to the future of marriage and the family in both France and the UK. There has been no warning or consultation with the people," he said.
"It is children and the future generations that will suffer most as a result of these unjust changes. I believe that gay people's request is sincere but children's rights and freedoms are too often forgotten in this debate about the 'rights of adults'.
"This is now a pan-European challenge and it needs a pan-European response."