Riot police have fought militant protesters in Rome as the biggest of a series of global rallies against banks and politicians tipped into violence.
At least 70 people were injured, three of them seriously, as police fought masked rioters with tear gas, water cannon and batons.
Other protesters tried to stop the rioters as they attacked cars and businesses, marring a peaceful rally.
The day saw coordinated protests in cities worldwide, most of them small.
Inspired by the Occupy Wall St movement and Spain's "Indignants", demonstrators turned out from Asia to Europe and back to New York for an event organisers said on their website was aimed at initiating "global change".
"United in one voice, we will let politicians, and the financial elites they serve, know it is up to us, the people, to decide our future," they added.
Tens of thousands of people had turned out to demonstrate peacefully in Rome.
Television pictures from the city showed streets packed with protesters waving banners, close to the Colosseum.
However militants dressed in black infiltrated the crowd and began attacking property. Offices belonging to the Italian defence ministry were set on fire, some cars were burnt including an armoured police vehicle, in addition to attacks on cash dispensers and bank and shop windows.
The militants were challenged by other protesters, the BBC's David Willey reports from Rome. "No to violence!" they shouted and tried to restrain them. The injured included at least 30 police officers.
There was a message of support for the global day of protest from the chief of the Bank of Italy, Mario Draghi, who is set to take over as head of the European Central Bank (ECB) next month.
"Young people are right to be indignant," he was quoted by Italian media as saying.
"They're angry against the world of finance. I understand them... We adults are angry about the crisis. Can you imagine people who are in their twenties or thirties?"
Outside the ECB itself in Frankfurt, Germany, thousands of people gathered to protest on Saturday.
A 27-year-old schoolteacher who gave his name only as Tobias told AFP news agency: "I see the global capitalist system as a time bomb for humans but also for the planet.
"Our well-being is financed to the detriment of other countries, [and] the ECB represents this unjust and murderous system."
Tens of thousands of people filled central Madrid on Saturday evening, the BBC's Sarah Rainsford reports from the Spanish capital.
People of all ages, from pensioners to children, and many of the young unemployed, gathered on Puerta del Sol Square, where the "Indignant" movement was launched in May. Organisers put the numbers at half a million.
In other developments
- Hundreds of protesters marched through New York and the streets of downtown Washington DC
- In Greece, about 2,000 people rallied outside parliament in Athens and a similar number reportedly turned out in the second city, Thessaloniki
- At least 1,000 people demonstrated in London's financial district but were prevented by police from reaching the Stock Exchange, and five arrests were made
- In Dublin, about 400 people marched to a hotel where an EU/IMF/ECB delegation involved in the country's ongoing financial bailout is staying, the Irish Times reports
- Hundreds of people marched in New Zealand cities while in Sydney, Australia, some 2,000 people - including representatives of Aboriginal groups, communists and trade unionists - rallied outside the central Reserve Bank of Australia
- "Occupy" protests were also held in South Korea, the Philippines, Taiwan and Hong Kong
Observers say that, while the original protesters in Spain had concrete demands such as seeking a cut in working hours to tackle unemployment, many "Occupy" protesters are vague in their demands.