The U.S. stood united behind France Saturday as the country reeled from the deadly terrorist rampage in Paris late Friday that left at least 129 dead and 352 others wounded.
As ISIS claimed responsibility for the coordinated assault, nations across the world, including the U.S., pledged solidarity with France, where President Francois Hollande declared three days of mourning and raised the nation's security to the highest level.
Secretary of State John Kerry said the U.S. stands ready "to provide whatever support the French government may require." He said the U.S. embassy in Paris is "making every effort to account for the welfare of American citizens in the city."
In the U.S., rallies and vigils were taking place to honor the victims and to send a message of support to France, still reeling from January's three-day terror spree that left 20 people dead.
A rally was being held in New York City's Washington Square Park on Saturday, to "send our love and support to the people of France and stand with them in these terrible times," according to "New York is Paris" Facebook page. Mayor Bill de Blasio attended the event and at a press conference following the rally urged New Yorkers to look out for each other.
"If you see something, say something," he said. "That phrase is real. It is powerful."
A candlelight vigil was also set to take place at the French Consulate in New York City from 7 to 9 p.m. ET.
The Metropolitan Opera in New York City sang France's National Anthem before the 1 p.m. matinee of "Tosca," in solidarity with France.
The French Consoluate in Los Angeles and the Islamic Center of Southern California held vigils on Friday. The organizers, Muslim and interfaith leaders condemned the attacks in paris as a violation of Islamic teachers.
In Boston, a makeshift memorial for victims was growing outside the French Consulate.
Some people left flowers on Saturday. One man also left a note saying, "We are all France. A grateful USA remembers," according to television reports.
Boston Police were also keeping watch outside the consulate. A rally of support was scheduled for Sunday at noon ET.
Outside the gate to the French Embassy in Washington, D.C., there were dozens of bouquets, signs of support and even a few bottles of wine.
"We love France," read one sign on Saturday. "Pray for France," read another, according to the AP.
Catherine Farvacque-Vitkovic, a Frenchwoman who has lived in the U.S. for 30 years, brought a bouquet of flowers in red, white and blue.
"We need to stand up for what we believe, and I hope that people will continue to assemble, not to fear," she said.
Gerard Araud, the French ambassador to the U.S., tweeted Saturday that the embassy has been allowed to hold a vigil at the statue of France's General Marquis Gilbert de Lafayette on Lafayette Square in D.C. at 5 p.m. ET.
In Miami, St. Thomas the Apostle Catholic Church held Holy Hour mass in French on Saturday in honor and memory of those who lost their lives in the terrorist attacks. A rally was also being organized in Miami Beach at 8 p.m. ET, according to the "Miami is Paris" Facebook page.
In cities across the world, people have been placing flowers at French embassies, including in Moscow and Montreal.
Meanwhile, a Facebook page created in the aftermath of the attacks called for a candle to be lit at "every window" to honor the victims.
"A light, a candle at every window," a message in French and in English said on the page. "In this day of mourning, facing horror and grief, let's show once more that we are here, standing up, together and united. We need to commemorate the memory of the dead, and send our thoughts and full support to the injured ones and their relatives. As night falls, let us light a candle at our windows. We are not afraid, we are together."
Pope Francis on Saturday told the Italian Bishops Conference TV2000 that the attacks in Paris were "part" of a "third World War," which is being waged through crimes, massacres, religious persecution and the destruction of cultural sites, according to the AP. He said "there are no justifications for these things."
In the U.K., two vigils were planned in London's Trafalgar Square in honor of those killed. The first event was planned for 9 a.m. ET and a larger gathering was expected at 2 pm ET.
Landmarks across the world glowed blue, white and red, the colors of the French flag, Friday night.
New York Gov. Andrew Cuomo has directed One World Trade Center to light up in blue, white and red "as we stand in solemn solidarity with the people of France, just as they have done for us in our own times of tragedy," he said in a statement.
The 408-foot spire on top of the building – constructed after Sept. 11, 2011 terror attacks – was lit up in the colors of the French flag Friday night, and will stay that way in the nights ahead.
The Empire State Building, meanwhile, went dark at 10 p.m. Friday in sympathy with Paris.
At New York's Union Square Park Friday night, dozens gathered to hold a candlelight vigil. They stood in a circle around a sign drawn on the ground, symbolizing both the peace sign and the Eiffel Tower, holding candles and the French flag.
Photos of the vigil were posted to social media with hashtags like #prayers4paris and #newyork4paris.
At the French Embassy in New York City, workers lowered the French flag outside the building where people left flowers. One woman was overcome with tears as she lit a candle to leave on the doorstep.
"First it gripped me. Now I'm angry and saddened and I want to pay homage to the French people, all the lives lost senselessly," said Theresa Thomas, who lives on the Upper East Side.
Vigils and rallies called "Chicago Stands with Paris" were held at the French consulate in Chicago at 6:00pm EST. On Sunday, the Alliance Francaise organized a "French in Chicago: Thoughts for Paris" vigil at 3:00 pm EST at the consulate.
San Francisco's City Hall, the Omni Hotel in Dallas, Toronto's CN Tower, for decades the tallest freestanding structure in the world, were also lit blue, white and red on Friday.
The Sydney Opera House also lit up in solidarity with France and more than 200 people gathered in Sydney's Martin Place, scene of the Lindt cafe siege, to hold a candlelight vigil to remember the victims of the Paris attacks.
London's Tower Bridge was illuminated in the colors of France's flag Saturday evening and so was Berlin's iconic Brandenburg Gate. Hundreds of people gathered Saturday on Paris Square, in front of the gate, in a show of solidarity with France, the AP reported.
But Parisian Mayor Anne Hidalgo tweeted out the Eiffel Tower will remain dark Saturday evening in mourning and in solidarity with the victims of the attack.
Danielle Abreu contributed to this report.