(CNN) -- Protesters filled Paris' Esplanade des Invalides on Sunday, rallying against a new law that will allow same-sex marriage in France starting Wednesday.
Paris police estimate 150,000 demonstrators marched along three different routes before converging in the sprawling plaza along the Seine River.
The arguments against same-sex marriage in France sound similar to those expressed in other countries, with many opponents saying they're for more civil rights, but against homosexuals officially getting married. Other protesters called themselves "a defender of the family," or said they "are not homophobic, (but) just want to protect children."
CNN's French affiliate BFM said 4,500 police were deployed Sunday to keep the peace after extremist threats, but the event proved to be calm.
On May 18, French President Francois Hollande signed into law a bill allowing same-sex couples to marry and adopt. The signing came a day after France's top court, the Constitutional Council, ruled that the bill adheres to the constitution.
France is the ninth country in Europe to allow same-sex marriage.
If pending legislation in New Zealand and Uruguay is enacted as expected this year, the worldwide total of countries with legalized same-sex marriage will rise to 14.
CNN's Laura Smith-Spark and journalist Saskya Vandoorne contributed to this report