In the French capital, where demonstrations were banned by the authorities after attacks by Islamic State militants killed 130 people on November 13, activists laid thousands of pairs of shoes in the Place de la Republique to symbolise absent marchers.
Among the high heels and sandals were a pair of plain black shoes sent by Pope Francis, who has been a vocal advocate for action to prevent dangerous climate change, and jogging shoes from UN secretary-general Ban Ki-moon.
About 10,000 people also joined arms to form a human chain through Paris along the 3km route of the banned march, organisers said.
“This is a moment for the whole world to join hands,” said Iain Keith, campaign director for Avaaz, one of the organisers.
Elsewhere, more than 2,000 events were being held in cities including Dublin, London, Sao Paulo and New York, making it perhaps the biggest day of climate action in history on the eve of the Paris summit, which runs from today to December 11 and will be attended by about 150 heads of government.
Around the world, activists marched, dressed as polar bears or penguins at risk from melting ice, or chanted slogans such as “climate justice”.
In Sydney, about 45,000 people are estimated to have marched through the central business district towards the Opera House. Protesters held placards reading: “There is no Planet B,” and “Say no to burning national forests for electricity”.
US president Barack Obama and China’s Xi Jinping will be among the leaders attending the start of the summit, which organisers hope will produce a first legally binding agreement to commit both rich and developing nations to curbing emissions of greenhouse gases, blamed for warming the planet, beyond 2020.
Hopes are high that the Paris summit will not fail like the previous climate summit six years ago in Copenhagen.
But all sides say pledges made in Paris will be insufficient to limit a rise in global temperatures to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial levels, widely viewed as a threshold for dangerous changes in the planet’s climate system.
Almost all the demonstrations were peaceful but, after the human chain protest in Paris, riot police fired tear gas and clashed with about 200 protesters, some wearing masks, in the Place de la Republique.
Demonstrators carried banners calling for the defence of the climate and democracy. The square has been a gathering place for Parisians since the November 13 attacks.
Using the state of emergency rules, police put 24 green activists under house arrest ahead of the summit, saying they were suspected of planning violent protests.
In Berlin, about 5,000 people marched with some dressed as penguins. One carried a huge “There is no planet B for penguins,” banner.
In London, an estimated 50,000 marchers were joined by fashion designer Vivienne Westwood and actress Emma Thompson. “This is our planet and we are in deep, grave danger,” Thompson told Sky television.