Trouble continued to flare late into the night as hundreds of people attempted to hijack the massive anti government cuts demonstration in central London.
Riot police fought activists in Trafalgar Square as violent protesters threatened to overshadow the TUC rally in Hyde Park which had earlier passed off peacefully.
A Metropolitan Police spokesman said between 200 and 300 people had gathered at the landmark location late on Saturday evening.
He said: "A large number from the crowd are throwing missiles and have attempted to damage the Olympic clock within the square. Officers have come under sustained attack as they deal with the disorder and attempted criminal damage."
In stark contrast, the daytime demonstration was hailed a "fantastic success" by trade unions as people from across the UK marched through central London.
Organisers estimated between 400,000 and 500,000 teachers, nurses, firefighters, council and NHS workers, other public sector employees, students, pensioners and campaign groups converged on the capital.
Union officials and Labour leader Ed Miliband condemned the "brutal" cuts in jobs and services. But during the good-natured protest hundreds of activists not connected with the union rally clashed with police in the West End. Officers were attacked as they tried to stop demonstrators smashing their way into banks and shops.
Paint, fireworks and flares were thrown at buildings, while the outnumbered police were attacked with large pieces of wood. Branches of HSBC, RBS, Santander and Topshop were among those to have their windows smashed.
The Met said 202 people had been arrested for a variety of offences including public order offences, criminal damage, aggravated trespass and violent disorder, but the number looked set to rise further. All those detained remained in custody at various London police stations.
TUC general secretary Brendan Barber said he "bitterly regretted" the violence, adding that he hoped it would not detract from the massive anti-cuts protest: "I don't think the activities of a few hundred people should take the focus away from the hundreds of thousands of people who have sent a powerful message to the Government today."