Inspired by the Occupy Wall St movement and Spain's "Indignants", demonstrators from Asia to Europe took to the streets. Riot police in Rome charged hundreds of protesters and fired water cannons, while a group of activists set alight a defence ministry annex nearby.
Flames could be seen coming out of the roof and windows of the building on Via Labicana as firefighters struggled to tame the blaze. Dozens of masked protesters could be seen in the area, which had not been cordoned off.
The violence was said to be caused by hooded militants known as "black blocks," who have infiltrated demonstrations in the past. There were no immediate reports of injuries.
Television images showed one of the cars in flames and spewing thick black smoke over the route of the demonstration, which was otherwise peaceful.
Police were out in force in Rome as some 100,000 protesters were expected a day after Premier Silvio Berlusconi barely survived a confidence vote.
Italy, with a national debt ratio second only to Greece in the 17-nation eurozone, is rapidly becoming a focus of concern in Europe's debt crisis.
In Frankfurt, police said 5,000 were in the square dominated by a giant blue-and-yellow euro logo.
Youths, retired people and families with children in strollers joined the protest in bright autumn sunshine, with signs reading "Smash the Dictatorship of Capitalism" and "Don't Sell Out Democracy at the ECB".
"I see the global capitalist system as a time bomb for humans but also for the planet," said a 27-year-old protester who gave his name only as Tobias.
"Our (material) well-being is financed to the detriment of other countries, (and) the ECB represents this unjust and murderous system," said the schoolteacher, carrying a sign reading "Capitalism Will Kill Capitalism."
Another protester, pensioner Christl, said she came thinking of her children and grandchildren "who will have to pay for our debts".
"I think the banking system is awful overall," she said.
"Investment banks should bear the cost of their losses on their own. It's time to rise up, otherwise nothing will ever happen."
Frauke Distelrath of the Attac anti-globalisation movement that organised the protest said she was pleased with the turnout.
"We chose to demonstrate outside the ECB, but we could have demonstrated outside Deutsche Bank, or outside the Frankfurt stock exchange," she said
Further afield, hundreds of people marched in the New Zealand cities of Auckland, Wellington and Christchurch.
In Australia, 2,000 people gathered in Sydney - including representatives of Aboriginal groups, communists and trade unionists - outside the central Reserve Bank of Australia.
