Facebook is helping users in Paris quickly post safety status messages after a series of shootings and explosions around the city killed more than 100 people Friday.

The social network has activated its Safety Check tool, which is automatically sending users in the affected area a note asking if they're safe. When a user clicks "Yes, let my friends know," the tool then notifies their Facebook friends.

Users outside Paris can also check to see if their friends are OK.

Enlarge Image Facebook; screenshot by CNET

Facebook determines location based on what users have listed in their profile, the city where they're using the Internet, and, if they're using the social network's Nearby Friends feature, their most recent location. If Safety Check has gotten the location wrong, users can mark that they're somewhere else.

Facebook's response comes as officials in Paris are urging residents to avoid going out due to one of the deadliest acts of violence in France in decades.

"Communication is critical in these moments," Facebook said in a statement, "both for people there and for their friends and families anxious for news."

Parisians are also turning to Twitter to get information and express solidarity, as well as to help those seeking safety from the attacks. Under the hashtag #PorteOuverte ("open door"), Twitter users in Paris have been posting their address to offer shelter.

Officials said more than 100 people were killed inside a Paris concert hall where attackers seized hostages. The hostage-taking episode was one in a series of at least six attacks across the French capital, according to CBS News.

Two French police officials said at least three attackers were killed when authorities launched an assault on the concert hall, where an American heavy metal band, the Eagles of Death Metal, were performing a sold-out show.

One official described the scene inside the building as "carnage" and said the attackers tossed explosives at the hostages.

Elsewhere in Paris, police said 11 people were killed in a restaurant and at least 3 people died when bombs went off outside a stadium north of Paris during an exhibition soccer match between France and Germany.

French President Francois Hollande was at the game and was evacuated from the stadium. Hollande later said he was closing the country's borders and declaring a state of emergency.

"It's a horror," he said.

Meanwhile in the US, President Barack Obama said in a briefing at the White House that his country stands behind the French and is ready to respond.

"This was an attack not just on Paris...not just on the people of France, but this is an attack on all of humanity and the universal values that we share," he said.