PARIS — Cartoonists, religious leaders and top French officials are paying respects to 17 people killed by Islamic extremists targeting satirical newspaper Charlie Hebdo and a kosher supermarket.

The attacks that started exactly four years ago Monday were the first of several to hit France, which remains on high alert for terrorist violence.

In solemn silence, mourners lay flowers at the entrance to Charlie Hebdo's offices in Paris, where 12 people were killed by two French brothers linked to al-Qaida.

Commemorations are continuing at the Hyper Cacher market, where a radical Frenchman who claimed allegiance to the Islamic State group held hostages and killed four people. He also killed a policewoman.

Government ministers and the mayor of Paris presided over Monday's events.

The attackers were killed in shootouts with police.