Paris riot police have fought back crowds who pushed their way on to the French capital's landmark Champs-Elysees avenue as part of a huge protest against a draft law allowing same-sex couples to marry and adopt children.

Hundreds of thousands of people, including conservative activists, children, retirees and priests converged on the city on Sunday in a last-ditch bid to stop the bill, many bused in from the French provinces.

The lower house of France's parliament approved the "marriage for everyone" bill last month with a large majority, and the measure faces a vote in the Senate next month.

Both houses are dominated by French President Francois Hollande's Socialist Party and its allies.

Sustained protests led by opposition conservatives in the traditionally Catholic country have eroded support for the draft law in recent months, and organisers hope Sunday's march will weigh on the Senate debate.

The first few hours of the protest were peaceful. But as it was meant to be winding down, about 100 youths tried to push past police barricades onto the Champs-Elysees, the avenue that cuts through central Paris and draws throngs of tourists daily.

In an indication of the sensitivity of the issue, protesters had been barred from marching on the avenue.

Police officers wrangled with the youths and then fired teargas to force them back. Gaining momentum, more and more protesters took side streets to reach the avenue, blocking a key intersection on the route to the president's Elysee Palace.

Anger and disappointment

Police fired more teargas but were unable to block the crowds from spilling on the avenue.

"Hollande, Resignation!" the protesters chanted, before breaking into the French anthem, "La Marseillaise".

For many people, the demonstrations have become outlets for anger and disappointment in Hollande's presidency.

An official with the Paris police headquarters said two people were arrested and no injuries were reported. The police official was not authorised to be publicly named in accordance with police policy.

Television footage showed scuffles breaking out, with security forces firing teargas on pink-clad marchers waving flags and chanting slogans against Hollande.

In France, anti-gay protesters often wear pink. Official police estimates put the turnout at around 300,000.

It was the second such protest this year after a similar march in January highlighted eroding public support for the bill that had forced deputies to put off a plan to allow lesbian couples access to artificial insemination,

Hollande has pledged to push through the law with his socialists' parliamentary majority and has angered opponents by trying to avoid public debate on the reform, which Justice Minister Christine Taubira described as a "change of civilisation".

Opponents of gay marriage and adoption, including most faith leaders in France, have argued that the reform would create psychological and social problems for children, which they believe should trump the desire for equal rights for gay adults.