Thousands of Spaniards have marched to mark the first anniversary of the grassroots "Indignados" movement that has sparked similar protests around the world.
Up to 600 people denouncing the Bank of England rallied in London and a Reuters witness said scuffles broke out between some demonstrators and police, with at least 12 arrests.
The Indignados and the offshoot Occupy and Take the Square movements had called for a global day of action against anti-debt austerity policies and the widening gap between rich and poor.
Demonstrators gathered in more than 80 cities across Spain, chanting the slogan that has become a mantra at protests over the past year: "They say it's democracy but it's not."
In central Madrid, streets were blocked as activists convened in various neighbourhoods across the capital to march towards Puerta de Sol, which filled up with people waving flags and chanting to the beat of horns and drums.
"The situation is getting worse but the root of the problem remains the same; this is a moment of crisis for capitalism," Jesus Gonzalez, 38, an airline employee said as he made for the Puerta del Sol.
Some 2,000 anti-riot police deployed to prevent protesters from setting up tents in the capital in a repeat of last year's camp-out.
Protesters vowed four days of demonstrations to inject fresh life into a movement that has suffered internal divisions.
The group behind the Puerta de Sol encampment last May - "Democracia real Ya!" (Dry), or Real Democracy Now - recently voted to register as a formal organisation.
In London, up to 600 people marched through the centre of the city, the number dwindling to around 200 after the demonstration reached its destination at the Bank of England.
Protesters erected 11 tents nearby and flew banners that read "Bank of England, the St Paul's of money," in reference to St Paul's Cathedral, from which a long-running Occupy tent encampment was evicted in February.
Occupy activists said they would dismantle the tents within hours and complained of police "aggression" and heavyhandedness. "We're under siege," said activist Ronan McNern.
In Moscow, a few hundred people camped by the central city pond in an Occupy-style protest over the police crackdown on a 6 May anti-Kremlin rally held ahead of the inauguration of President Vladimir Putin.
