PARIS — Several hundred thousand people massed at the Eiffel Tower in Paris on Sunday to protest a plan by President François Hollande to legalize gay marriage and adoption.

Three columns of protesters, waving pink and blue flags showing a father, mother and two children, converged from different meeting points in Paris. Many came after long train and bus rides from the provinces.

Mr. Hollande has pledged to push through the law with his Socialists’ parliamentary majority but a campaign by opponents has dented public support and forced deputies to put off a plan to allow lesbian couples access to artificial insemination.

The park at the Eiffel Tower was packed, but turnout estimates varied widely. Organizers claimed 800,000 had protested, while police put the number at 340,000, high even in protest-prone France.