Thousands of demonstrators are preparing to take part in a march for "jobs, justice and climate" amid anger at the £19m cost of staging the G20 summit, ahead of a week of protests to mark the conference.
Foreign Office minister Lord Malloch-Brown disclosed the figure, adding that a lot of the money would be spent on security because of the number of high-profile leaders travelling to the capital for the event on 2 April.
Organisers of some of the dozens of protests being held to coincide with the summit criticised the cost.
Leave for police in the capital has been cancelled for the summit amid warnings from Scotland Yard that some campaigners were planning disruption around the city's Square Mile.
A professor, thought to be masterminding "mayhem" for next week, was suspended by his university after he warned bankers could be "hanging from lampposts".
The University of East London confirmed a "full investigation" was launched following warnings of violence from Chris Knight, 66, from Lewisham, south London.
The Put People First march will be held in central London tomorrow, organised by an unprecedented alliance of 150 groups, including unions, charities, environmental campaigners, development and faith organisations.
Thousands will march to Hyde Park for a rally with speakers and entertainment, including Luke Pritchard, from the Kooks, comedian Mark Thomas and Mauritanian musician Daby Touré. An ecumenical church service will feed into the march from Central Hall Westminster.
Paul Kenny, leader of the GMB union, said today: "GMB wants jobs, public services, no global poverty or inequality and a green economy to slow climate change. If the G20 listens and begins to take action GMB will see it as progress."
Glen Tarman, who has helped organise the march, said he had no evidence anyone intended to break the law or commit acts of violence.
Lindsey German, convener of the Stop The War Coalition, which is organising a series of events over the next week, said: "Many millions of pounds are being spent of policing, closing roads, travel and bringing entourages to the UK, but the politicians are fiddling while Rome burns.
"We are in the middle of one of the worst recessions ever, the war in Afghanistan is raging, the people of Gaza continue to suffer, but none of these will be tackled at the summit."
Murray Benham, head of campaigns at the World Development Movement, said: "The cost of the summit is clearly incredibly high. The only possible justification for this expenditure will be if the G20 leaders make the bold moves necessary to make the global economy work for people in the developing world, who are being hit the hardest by the credit crunch and climate change.
"The fear is that if the summit is just a talking shop, then this opportunity and money will have been squandered, and that would be disastrous for the world's poorest people."
Meanwhile, economists warned the need for global co-ordination and co-operation was vital, but there were fears there was little the G20 meeting could achieve, with countries already buckling under the strain of fiscal measures taken so far, and a clear stand-off emerging between the UK/US and other major economies.
Experts have said one of the most important outcomes would be for a greater commitment to encourage free trade and to promote the fight against protectionism, which risks exacerbating the recession.