Police are on alert in Paris ahead of a large rally against the new same-sex marriage and adoption act, with 50 arrests on the eve of the protest.
Thousands of people have gathered at three points around the city to march to the Invalides complex in the centre, where they will meet on the esplanade.
It is the first big rally since the law was signed on 18 May.
Earlier protests attracted tens of thousands of people and spilled into clashes with police.
The 50 people arrested had staged a protest which blocked the French capital’s famous Champs-Elysees avenue.
Members of the conservative UMP opposition are backing Sunday’s event.
France is now the ninth country in Europe, and 14th globally, to legalise gay marriage.
But French people have been bitterly divided over the issue. On Tuesday, a far-right historian shot himself dead in Notre Dame Cathedral, leaving messages in which he denounced gay marriage.
Police expect some 200,000 people to attend Sunday’s demonstration, including hundreds of potential troublemakers.
Among those due to take part are members of a radical new movement called French Spring (French: Printemps Francais), which the interior ministry has threatened to dissolve due to its inflammatory rhetoric.
On Saturday evening, a group of protesters chained themselves to metal barriers they had placed in the middle of the Champs-Elysees.
Some released smoke bombs before police moved in and arrested them. Nobody was hurt.
Mr Myard told the BBC there was a “huge gap between this government and the citizens” over the marriage issue.
“This is something we cannot accept because of the fate of the children,” he said.
“Those people are playing God, because they want to marry, but it won’t stop at this stage. Then they will adopt, and then we will have children in families where there is no father or no mother.”
Opposition to gay marriage has become conflated with all sorts of other anti-government grievances coming from the right and the atmosphere in the country is particularly volatile, the BBC‘s Hugh Schofield said in a recent report.