Up to 40,000 protesters are set to march through central London ahead of the G20 summit of world leaders.
Activists and campaigners from around the world were gathering in the city to take part in a series of protest rallies, marches and stunts throughout the week which police fear will degenerate into violence.
The series of events will start with a march for “jobs, justice and climate”, organised by an alliance of more than 150 unions, environment, charity, faith and development groups.
Tens of thousands of climate change activists, anti-war campaigners, trade unionists, charity workers and students are expected to join forces in the march.
Their protest will target bankers and financiers they blame for the worst global recession of the past 50 years.
TUC general secretary Brendan Barber, due to speak at a huge rally in Hyde Park, will say: “It is right to be angry as there is nothing inevitable about this recession … Today is the birth of a powerful progressive voice.”
Thousands of police officers will be on duty in central London in the days preceding the summit on Thursday and to deal with any violence in the run-up to the conference.