Wrigley Field's marquee was shut off, and the Sears Tower was one of 800 buildings in the city to go dark. Earth Hour began at 8:30 p.m. and lasted until 9:30 p.m.
Communities throughout Illinois planned to dim their lights as part of the World Wildlife Fund event. Nearly 4,000 cities and towns in 88 countries are participating.
Chicago is one of Earth Hour's 10 US flagship cities.
"No matter what your individual beliefs are about climate change, energy efficiency is something everyone can understand in this economic environment," said WWF managing director Darron Collins, who helped Chicago officials organize for the night.
Some building managers in Chicago are taking the idea of Earth Hour beyond just one day. Many plan to shutdown signs or decorative lighting after 10 p.m. on weekdays and 11 p.m. on weekends.
Last year, more than 2.7 million residents turned off their lights for Earth Hour, which was started by the World Wildlife Fund's Global Campaign. The hope is to raise awareness about emergency efficiency and global climate change.
Rolling Darkness Around The Globe
From an Antarctic research base and the Great Pyramids of Egypt to the Empire State Building in New York, illuminated patches of the globe went dark Saturday for Earth Hour, a campaign to highlight the threat of climate change.
Time zone by time zone, nearly 4,000 cities and towns in 88 countries joined the event sponsored by the World Wildlife Fund to dim nonessential lights from 8:30 p.m. to 9:30 p.m. The campaign began in Australia in 2007 and last year grew to 400 cities worldwide.
Organizers initially worried enthusiasm this year would wane with the world focused on the global economic crisis, said Earth Hour executive director Andy Ridley. But he said it apparently had the opposite effect.
"Earth Hour has always been a positive campaign; it's always around street parties, not street protests, it's the idea of hope, not despair. And I think that's something that's been incredibly important this year because there is so much despair around," he said.
Crowds in Times Square watched as many of the massive billboards, including the giant Coca-Cola display, darkened. Steps away, the Majestic Theater marquee at the home of "The Phantom of the Opera" went dark, along with the marquees at other Broadway shows. Mikel Rouse, 52, a composer who lives and works nearby came to watch what he called "the center of the universe" dim its lights.
"C'mon, is it really necessary? ... All this ridiculous advertising ... all this corporate advertising taking up all that energy seems to be a waste," Rouse said.
The Smithsonian Castle, World Bank, National Cathedral and Howard University were among several buildings that went dark for an hour in the nation's capital.
"This was the first year that Washington, D.C., became an official Earth Hour city," said Leslie Aun, WWF spokeswoman.
In the Chilean capital of Santiago, lights were turned off at banks, the city's communications tower and several government buildings, including the Presidential Palace where President Michelle Bachelet hosted a dinner for U.S. Vice President Joseph Biden.
The two leaders and dozens of guests dinned at candlelight.
In San Francisco, lights on landmarks like the Golden Gate Bridge were set to be turned off, along with the city's well-known Ghirardelli Square sign. The Las Vegas Strip will turn down its glitz by extinguishing the marquees and decorative lighting outside casinos, as well as the famous "Welcome to Fabulous Las Vegas" sign.
The honkytonks in Nashville went dark as country music stars Jo Dee Messina and Big Kenny Alphin of the duo Big & Rich entertain a crowd with a free concert.
"I think it's fascinating that so many cities are taking part and that something as simple as shutting off the lights can make such a difference. It's something everyone can do," Messina told the AP.
Brenda Sanderson, owner of four Nashville honkytonks, said she expects the crowd to be surprised, but noted Nashville is also known for acoustic music.
"We're going to do it acoustic for a while and let the crowd play along and see if they enjoy it," Sanderson said.
U.N. Secretary Ban Ki-moon called Earth Hour "a way for the citizens of the world to send a clear message: They want action on climate change."
An agreement to replace the Kyoto Protocol, which expires in 2012, is supposed to be reached in Copenhagen, Denmark, this December, and environmentalists' sense of urgency has spurred interest in this year's Earth Hour.
In Bonn, WWF activists held a candlelit cocktail party on the eve of a U.N. climate change meeting, the first in a series of talks leading up to Copenhagen. The goal is to get an ambitions deal to curb emissions of heat-trapping gases that scientists say are dangerously warming the planet.
"People want politicians to take action and solve the problem," said Kim Carstensen, director of the global climate initiative for WWF, speaking in a piano bar bathed by candlelight and lounge music.
China participated for the first time, cutting the lights at Beijing's Bird's Nest Stadium and Water Cube, the most prominent 2008 Olympic venues. In Bangkok, the prime minister switched off the lights on Khao San Road, a haven for budget travelers packed with bars and outdoor cafes.
Earth Hour organizers say there's no uniform way to measure how much energy is saved worldwide.
Earth Hour 2009 has garnered support from global corporations, nonprofit groups, schools, scientists and celebrities -- including Oscar-winning actress Cate Blanchett and retired Cape Town Archbishop Desmond Tutu.
McDonald's Corp. planned to dim its arches at 500 locations around the U.S. Midwest. The Marriott, Ritz-Carlton and Fairmont hotel chains and Coca-Cola Co. also planned to participate.
Associated Press Writers around the world contributed to this report.
On the Net:
Earth Hour: http://www.earthhour.org