Up to 1.6m are estimated to have taken to the streets of the French capital.

More than 40 world leaders joined the start of the Paris march, linking arms in an act of solidarity.

The marchers wanted to demonstrate unity after the attacks on satirical magazine Charlie Hebdo, police officers, and a kosher supermarket.

The rally, led by relatives of the victims of last week's attacks, began at the Place de la Republique and concluded in the Place de la Nation.

Several other French cities also held rallies. The interior ministry said turnout across France was at least 3.7 million, including up to 1.6 million in Paris - where sheer numbers made an exact tally difficult.

Rallies also took place outside of France, with thousands of people gathering in London, Washington, Montreal and Berlin.

In Madrid, several hundred Muslims held banners saying "Not in our name" next to the train station where in 2004 Islamist bombings killed nearly 200 people.

World leaders, including British Prime Minister David Cameron, German Chancellor Angela Merkel, Israeli Prime Minister Benjamin Netanyahu, Palestinian President Mahmoud Abbas, Malian President Ibrahim Boubacar Keita, EU President Donald Tusk, and Jordan's King Abdullah II joined the beginning of the Paris march.

"Paris is the capital of the world today," French leader Francois Hollande said.

The leaders observed a minute's silence before the march began.

About 2,000 police officers and 1,350 soldiers - including elite marksmen on rooftops - were deployed in the capital to protect participants.