Three columns of protesters, waving pink and blue flags showing a father, mother and two children, converged on the landmark from different meeting points. Many had taken long train and bus rides from the provinces.
Mr Hollande has pledged to push through the law with his Socialists' parliamentary majority but his opponents' campaign has dented public support and forced deputies to put off a plan to allow lesbian couples access to artificial insemination.
Champ de Mars park at the Eiffel Tower was packed, but turnout estimates varied widely. Organisers claimed 800,000 had protested, while police put the number at 340,000, high even in protest-prone France.
"Nobody expected this two or three months ago," said Frigide Barjot, a flamboyant comedian leading the "Demo for All". At the rally, she read out a letter to Mr Hollande asking him to withdraw the draft bill and hold an extended public debate on the issue. Backed by the Catholic Church hierarchy, Ms Barjot and groups working with her mobilised church-going families ,political conservatives, Muslims, evangelicals and even homosexuals opposed to gay marriage to protest.
Mr Hollande's office said the turnout was "substantial" but would not change his determination to pass the reform.
Meanwhile, the Vatican underlined opposition to gay adoption as same-sex marriage supporters staged a protest in St. Peter's Square. While the Pope was speaking four women took off their tops to show the slogan "In Gay we Trust" on their bodies.