US Vice-President Joe Biden has called for G20 protesters to give governments a chance to tackle the economic crisis.
At a G20 warm-up meeting in Chile, Mr Biden said heads of state would agree proposals to remedy the crisis at next week's meeting in London.
As they spoke, tens of thousands of protesters marched in the UK capital and in Germany, France and Italy.
US billionaire George Soros told the BBC the G20 meeting was "make or break" for the world economy.
"Unless they do something for developing world there will be serious collapse in that part of the world," Mr Soros said.
Massive security operation
At a news conference in Vina del Mar, Mr Biden said he hoped the protesters would give the politicians a chance.
"Hopefully we can make it clear to them that we're going to walk away from this G20 meeting with some concrete proposals," he said.
British Prime Minister Gordon Brown said he understood why people were demonstrating in the UK.
"We will respond to [the protest] at the G20 with measures that will help create jobs, stimulate business and get the economy moving," he said.
But Brazil's President Luiz Inacio Lula da Silva told the Chile meeting that everyone was suffering from the recklessness of those who had turned the world economy into "a gigantic casino".
"We are rejecting blind faith in the markets," he said.
In London on Saturday, demonstrators demanding action on poverty, jobs and climate change called on G20 leaders to pursue a new kind of global justice.
Police estimated 35,000 marchers took part in the event.
A series of rallies are planned for Wednesday and Thursday by a variety of coalitions and groups campaigning on a range of issues from poverty, inequality and jobs, to war, climate change and capitalism.
There have been reports that banks and other financial institutions could be targeted in violent protests.
British officials have put a huge security operation in place.
'We won't pay'
Before the London summit, Mr Brown has been visiting a number of countries trying to rally support for his economic plans.
In Chile on Friday he said people should not be "cynical" about what could be achieved at the summit, saying he was optimistic about the likely outcome.
But in an interview, German Chancellor Angela Merkel dampened expectations of a significant breakthrough.
She said one meeting would not be enough to solve the economic crisis and finish building a new structure for global markets.
In Berlin, thousands of protesters took to the streets on Saturday with a message to the G20 leaders: "We won't pay for your crisis."
Another march took place in the city of Frankfurt. The demonstrations attracted as many as 20,000 people.
In the Italian capital, Rome, several thousand protesters took to the streets.
In Paris, around 400 demonstrators dumped sand outside the stock exchange to mock supposed island tax havens.