New York lit One World Trade Centre, the tallest building in America, red, white and blue in solidarity with France after attackers killed at least 140 people in Paris.
It was a powerful symbol of US friendship and support for the French, made from the tallest building in the US, which was built on the site of the 9/11 attacks on New York.
Its 124-metre spire is so large it can be seen across much of America's largest city and across the Hudson River in New Jersey.
"Today, and in the days ahead, New York will light One World Trade Centre in blue, white and red as we stand in solemn solidarity with the people of France," Governor Andrew Cuomo said.
"We join them in mourning those who were killed and in praying for those who were injured or lost loved ones. And we continue to stand side by side with them in our commitment to a free and peaceful world."
The French attacks are among the deadliest in the West since the September 11, 2001 terrorist attacks on New York, Washington and Pennsylvania.
Other US buildings also lit themselves in the colors of the French flag, including city hall in San Francisco and the Omni Hotel in Dallas, Texas.
Elsewhere in the city, French expats gathered in Union Square to express their solidarity with their compatriots.
Some of the 50 or so demonstrators carried handmade signs reading "Don't be afraid," and "Pray for Paris," as they stood together in silence.
"It's very hard not to be in Paris at this moment," said Anais Bourrut-Lacouture, 22, a business student.
Celebrated guerrilla artist Banksy on his Twitter account made a plea for "Peace for Paris, posting a painting in which the iconic lines of the Eiffel Tower were fashioned into a peace symbol. The image was re-tweeted thousands of times.
New York police meanwhile were on heightened alert, deploying counter-terrorism reinforcements to crowded places and outside the French mission to the United Nations, and the French consulate. The New York police department said the reinforcements were made "out of an abundance of caution" to reassure the public.
In a similar tribute, in Sydney the iconic sails of the Sydney Opera House will be lit in red, white and blue today as Australians express their solidarity with France after series of bloody attacks, officials said.
New South Wales state Premier Mike Baird said he had also asked for a French flag to fly over the city's famous harbour bridge. "Turns out, there isn't one in the country that is big enough," he wrote on Facebook. "So, if we can't source one, we are going to make one."
"We mourn with you, and we stand with you, Paris," he said, after gunmen shouting "Allahu akbar" massacred scores of diners and concert-goers across Paris on Friday night.
Earlier Australian Prime Minister Malcolm Turnbull had said that protecting freedom was a global battle after the attacks left more than 120 people dead.
He said it was a battle against those who seek to suppress freedom and seek to assert "some form of religious tyranny; a threat in the name of God but is truthfully the work of the devil".
"In France and Australia, all around the world, we stand shoulder to shoulder with the people of France and with all free peoples in the battle against terrorism," Turnbull said in a statement.
Here are some monuments and iconic buildings from cities across the globe that lit up in the colours of the France flag, as a symbol of solidarity:
New York:
NYC stands with France! #empirestatebuilding #prayforparis pic.twitter.com/d4qxJy5ho4
— Rafaholics™ (@Rafaholics) November 14, 2015
To the Washington Capitals, thank you. Our heart goes out to those in Paris tonight. #PrayersForParis pic.twitter.com/LSCdLPaV5t
— Gino Hard (@Ginohard_) November 14, 2015
The Dallas Omni Hotel is in full color representing all the Victims in Paris today. #prayforparis pic.twitter.com/HoYoS2YK18
— Daph (@ikonjeon) November 14, 2015
The convention centre in #dublin now showing solidarity with our French friends! A touching gesture. #ParisAttacks pic.twitter.com/IMTsH62ciO
— Rory Prevett (@therevprev) November 14, 2015
Toronto's CN Tower now lit in red, white and blue. Photo by @trueblue8209 #ParisAttack pic.twitter.com/ZimudZanlY
— Simon Ostler (@SimonOstler) November 14, 2015
From #UK; The Wembley Arch in French colors tonight #Paris #France pic.twitter.com/RsFzqBnfiS
— Rami (@RamiAlLolah) November 14, 2015
We're in this together. Brazil is praying for France! #PrayForParis pic.twitter.com/pUPk3ULeZu
— Kian and Jc Brasil (@KianAndJcBrasil) November 14, 2015
San Francisco stands with Paris. #tricolore pic.twitter.com/2peQhivpFT
— Jim Prosser (@jimprosser) November 14, 2015
Sydney Opera House lit up in French colours (pic via @Tomharling) #ParisAttacks pic.twitter.com/FwMKXywuS7
— Simon Cullen (@Simon_Cullen) November 14, 2015