Four people have been detained on suspicion of carrying out an attack at a Lille gay bar, the Interior Ministry said Thursday, amid nationwide tensions over a bill that would legalize gay marriage.

Several other people were detained in Paris late Wednesday after a protest against gay marriage that ended with some demonstrators fighting police and damaging cars along the Champs-Elysees avenue.

Legalizing gay marriage was one of President Francois Hollande's campaign promises, and polls show a majority of voters support the idea, as an increasing number of governments open marriage to same-sex couples.

But opposition to the bill has mounted throughout the French legislative process, largely from conservative groups from France's conservative heartland. While the protests are largely peaceful, violence has occasionally erupted on the sidelines.

At the same time, gay rights groups say they are seeing an increasing number of anti-gay attacks in recent weeks and months.

Hollande urged calm, expressing concern Thursday about "homophobic acts, violent acts" related to the gay marriage bill while insisting he respects "the right to protest."

Interior Minister Manuel Valls took a sterner tone, saying in a statement Thursday that he "condemns, with the greatest firmness, the homophobic aggression perpetrated last night in a bar" in Lille. He said the four suspects "clearly belonged to the extreme right movement" and are accused of intentionally targeting gay customers in the bar, punching the bar manager and causing material damage.

In a first reading, both houses of the French parliament have approved the bill that would legalize gay marriage and adoption by same-sex couples. The second reading is taking place this week.