New York has lit up the One World Trade Center in red, white and blue in solidarity with France, after attackers killed at least 150 people in Paris.
It was a powerful symbol of US friendship and support for the French, made from the tallest building in the United States, 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 Center in blue, white and red as we stand in solemn solidarity with the people of France," Governor Andrew Cuomo said.
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 have also lit themselves in the colours of the French flag, including city hall in San Francisco and the Omni Hotel in Dallas, Texas.
New York police are 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 says the reinforcements have been made "out of an abundance of caution" to reassure the public.