BARCELONA, Spain — Half a million people gathered in the heart of Barcelona on Saturday, clutching roses and holding banners denouncing violence and extremism, but also bearing signs warning against Islamophobia and calling on Spanish leaders to stop selling weapons.

The mass demonstration followed two attacks that killed 15 people last week, including 13 who were mowed down by a van that zigzagged down Las Ramblas, the most famous promenade in Barcelona.

On Saturday, marchers carried banners bearing the defiant message “No tinc por” — Catalan for “I’m not afraid.”