The rallies kicked off in Australia, where people affected by recent devastating wildfires joined young environmentalists protesting against the government's pro-coal stance.
Janet Reynolds said she had come to the rally in Sydney after losing everything in an "inferno, an absolute firestorm that raced through my property."
"It's so unnatural that I started investigating, reading science and really exploring what's happening with climate change," she told Australian television.
Student Daisy Jeffrey said protesters had come to help raise money for those affected by the fires and to demand action from the government
"People have lost their homes, people have lost their lives. We have to ask: How far does this have to go before our government finally takes action," she said.
Teenage activist Greta Thunberg, who is traveling across the Atlantic by sailboat to attend the climate talks, sent a message of support to protesters. "Everyone's needed. Everyone's welcome. Join us," she said on Twitter.
Since starting her one-woman "climate strikes" in Sweden more than a year ago, Thunberg has drawn a huge following around the world and inspired thousands more students to regularly skip school on Fridays and join climate protests.
Further rallies took place in Germany, Hungary, Belgium, South Korea, Poland, England, Turkey, Italy, Spain and France — where environmental protesters took a swipe at Black Friday.
In Berlin, about two dozen environmental activists jumped into the chilly waters of the Spree river in front of parliament to protest a government-backed package of measures they say won't be enough to reduce the country's greenhouse gas emissions. The package was blocked Friday by Germany's upper house, which represents the country's 16 states.
Later, tens of thousands of students rallied in front of the Brandenburg Gate.
"The generations before us messed it up," said 17-year-old Robin Ebelt. "And we're the ones that will feel the consequences. I would like to spend another 60 years on this planet, grow old and have grandchildren."
Quang Paasch of the activist group Fridays for Future said governments attending next week's annual climate conference should keep in mind the goals of the 2015 Paris accord, which set a target of keeping global warming well below 2 degrees Celsius (3.6 Fahrenheit). "We need to keep taking to the streets, we need to defend Paris."
Thousands of demonstrators also marched in Skopje, the capital city of North Macedonia, protesting high levels of air pollution, among the worst in Europe.
Organizers blamed the government for the weak implementation of safety standards that has led to some 3,500 deaths annually due to the exposure to harmful chemicals in the environment, according to United Nations health data.
In South Africa, a few dozen people holdings signs saying "Not Cool" and "Stop Pollution Now" protested outside the Johannesburg Stock Exchange in the summer heat of the Southern Hemisphere.
One protester lay on the ground faking death, holding a sign saying "Black Friday Reason to Grieve."
Africa contributes least to climate change and is the least prepared to deal with it. Temperatures in parts of the continent are projected to rise more quickly than the global average.
"The reality is that we have a climate change emergency," protest organizer Elana Azrai said. She noted water shortages in parts of the country amid a drought in southern Africa.
Elsewhere, officials have raised the alarm over unusually severe rainfall in East Africa and a pair of cyclones that ripped into Mozambique within weeks of each other early this year.
Scores of young Nigerians marched in downtown Lagos displaying messages such as "There is no planet B" and "Stop Denying the Earth is Dying" as passing vehicles slowed and honked in support.
"Mother nature is lamenting and we are grieved," declared one of the Lagos marchers, Omobolanle Eko. "The rise in temperature is real. The rise in sea level is real."
Student Folashade Gbadeola listed several possible solutions, some of them challenging, in Nigeria, whose economy is still deeply dependent on oil production.
"We should stop the use of fossil fuel," Gbadeola said.
And in a city of some 20 million people and epic traffic jams, the student suggested that people live near their place of work, ride bikes and share car rides.
The megacity is Africa's most populous and is among its coastal cities threatened by rising sea levels.