Tens of thousands of people are attending a rally and march in central London against cuts to public spending.
Marchers set off from Victoria Embankment to Hyde Park.
The Trades Union Congress, which organised the event, says 250,000 people are taking part, but the Metropolitan Police says it is not estimating numbers.
The BBC reports the march passed peacefully. But splinter groups attacked shops and banks and a stand-off with police took place in Piccadilly. There were 16 arrests.
Government ministers say the cuts are necessary to get the public finances in order. The budget deficit is currently running at about 10% of GDP.
The event is described as the biggest demonstration in the capital since the anti-war protests before the invasion of Iraq in 2003.
The budget was delivered by Chancellor George Osborne on Wednesday.
The cuts were previously announced via an emergency budget and spending review last year.