Extinction Rebellion is demanding that governments drastically cut the carbon emissions that scientists have shown cause devastating climate change.
They are backed by Ms Thunberg, the Swedish teenager whose searing UN address in September made international headlines, and by academics studying the rising temperatures and sea levels.
Their protests have irritated drivers and some officials but raised the hopes of those who see climate change as a threat to the planet.
In London, where the Extinction Rebellion movement was born last year, demonstrators put up structures on Westminster Bridge in the shadow of Britain's parliament.
Police had made 217 arrests by 17:15 pm (1615 GMT).
"Getting arrested sends a message to the government that otherwise law-abiding citizens are desperate," IT consultant Oshik Romem, from Israel but working in Britain for 19 years, told AFP while sitting on a road outside parliament.
'Running out of time'
Hundreds of Australians joined a sit-in on a busy inner Sydney road before being dragged away by the police. Thirty people were later charged.
"We have tried petitions, lobbying and marches, and now time is running out," Australian activist Jane Morton said.
Australia's conservative government has resisted adopting new environmental standards and backed lucrative coal exports.
Protests occurred in 60 cities around the world, including New Delhi, Cape Town, Paris, Vienna, Madrid and Toronto.
At New York's Battery Park, some 200 demonstrators took part in a "funeral march" to Wall Street, where protesters threw fake blood over the financial district's famous bronze statue of a bull.
"We need imagery like this in order to get people's attention," 29-year-old James Comiskey told AFP, as he carried a cardboard coffin in the procession.
Police arrested around a dozen people who staged a sit-in by the bull.
Campaigners in Dublin wheeled in a pink yacht and parked it outside the office of Irish Prime Minister Leo Varadkar.
"We are running out of time," Dutch protester Shirleen Chin told AFP in Amsterdam.
Dutch police said they arrested 90 people after Extinction Rebellion members occupied a bridge outside the famed Rijksmuseum art gallery.
The movement is partially credited with pushing the UK government in June to become the first in the Europe Union to commit itself to a net-zero target for harmful emissions by 2050.
Extinction Rebellion is demanding governments reach that target by 2025, as well as holding "citizens assemblies" to decide on policies to achieve that aim.
The parliament in Norway, not an EU member, in June adopted a target of 2030.
There has been less movement in other parts of Europe or the most impacted cities of Asia.
And not everyone out on the streets was impressed with the campaign.
"They're taking it out on everyday people trying to go about their business. They should go after big people," London taxi driver Dave Chandler told AFP.
Extinction Rebellion counters that emergencies like the one heating up the climate demands action from everyone across the world.
Hundreds barricaded themselves inside a Paris shopping center for hours over the weekend.
Groups unfurled banners with slogans such as "Burn capitalism, not petrol" above Paris restaurants and fashion boutiques.
And hundreds brought blankets and sleeping bags to one of the main roundabouts in central Berlin which police expect to be shut down for many days.
Extinction Rebellion's tactics in Australia prompted senior conservative politicians to call for protesters' welfare payments to be cut.
Sydney assistant police commissioner Mick Willing accused protesters of putting themselves and others at risk, warning that such disruptive protests in the future would "not be tolerated".