Violence has flared in Rome as protests against corporate greed and austerity measures spread to cities around the world.
The Occupy Wall Street movement, which started in New York, has sparked similar demonstrations in cities across the globe.
In Italy's capital up to 200,000 protesters took to the streets.
Police fired tear gas and used water cannons after shop windows were smashed and cars set on fire in isolated incidents.
In London, supporters of the Occupy London Stock Exchange (OccupyLSX) group held a rally outside St Paul's Cathedral.
Around 3,000 people gathered in the capital as part of the worldwide demonstrations.
Part of a "global movement for real democracy" to highlight social and economic injustice , protesters marched in the city's financial centre.
Police cordoned off Paternoster Square, where the Stock Exchange is located, as several hundred activists tried to march in.
Outside St Paul's, Wikileaks founder Julian Assange was one of several people who addressed the crowd.
"What is happening here today is a combination of dreams from many people all over the world," he said.
Previously, protests in London have turned into cat-and-mouse-type chases with demonstrators and police officers around the streets.
On Saturday though, the vast majority took part in debates discussing solutions to the economic crisis.
Protester James Meadway told Sky News: "This is about a democracy that is coming from the bottom up.
"We have drawn great inspiration from the Wall Street demos and this shows it can happen here too."
There were scuffles with police officers around the edges of the demonstration but nothing compared to the violence that broke out at protests in Rome.
The Facebook page of OccupyLSX has more than 13,000 followers, with more than 5,000 confirmed attendees.
In a statement the group said: "The words corporate greed ring through the speeches and banners of protests across the globe.
"After huge bailouts and in the face of unemployment, privatisation and austerity, we still see profits for the rich on the increase."
In Europe, the movement joined up with anti-austerity protests that have raged for months across the continent.
In Frankfurt, some 5,000 people took to the streets to protest in front of the European Central Bank.
Hundreds marched through the Bosnian city of Sarajevo carrying pictures of Che Guevara and old communist flags that read "Death to capitalism, freedom to the people".
Hundreds of people also joined peaceful protests in Sydney, Tokyo, Manila, Hong Kong and Seoul.