G7 finance ministers are to speak this weekend amid concerns of further turmoil in the global financial markets in the wake of the US credit rating downgrade.
Credit ratings agency Standard and Poor's said early on Saturday it had downgraded the country from its top AAA rating to AA+.
The loss of the top-notch rating could reignite panic on the markets with traders fearing the world's biggest economy may be heading back to recession.
Markets around the globe have suffered huge falls this week, although the US Dow Jones ended higher on Friday after better-than-expected employment figures.
In London, the FTSE 100 index of leading UK shares closed the day at 5246.99, down 146 points or 2.71%.
More than £148bn was wiped off the FTSE's value last week - a plunge of 568.2 points or 10.15% - caused by the eurozone debt crisis and fears the UK economy is stalling.
But in the US, the Dow Jones Industrial Average rose 0.54% at the close following volatile trading. The broader S&P 500 (SNP: ^GSPC - news) dipped 0.06%, while the technology-based Nasdaq Composite (Nasdaq: ^IXIC - news) fell 0.94%.
Over the week - the worst for American markets in more than two years - the Dow (NYSE: DPD - news) fell by a total of 5.8%, the S&P 500 was down 7.2% and the Nasdaq (Nasdaq: ^NDX - news) was off 8.1%.
The Nikkei index on the Tokyo Stock Exchange has slumped by 5.4% since last Monday.
Worries that Italy and Spain may need a bailout have sparked a new crisis in the eurozone.
In a bid to restore confidence Italian PM Silvio Berlusconi has pledged to balance the country's budget in 2013, a year earlier than planned.
But he has ruled our early elections in 2012 to stem panic in the markets, saying this was not an option.
"This has absolutely not been talked about," he told reporters in Rome. "This has never been an option."
Spain says its prime minister Jose Luis Zapatero and French President Nicolas Sarkozy have agreed on the need for a co-ordinated response from eurozone members to global concerns about its future.
News that 117,000 new jobs were created in the US last month brought some comfort to investors there on Friday before panic returned to the markets amid concerns of slower global growth.
US analysts said the sell-off was driven by fears over the European debt crisis and anxiety about slow growth, high unemployment and falling consumer confidence and spending.
IG Index analyst David Jones (Xetra: 898370 - news) said: "There is a growing sense that there is real confusion over how to deal with underlying problems... the slowing pace of recovery, threat of recession and eurozone contagion spreading.
"On 7 July, only 29 days ago, the FTSE 100 (Euronext: VFTSE.NX - news) was etching just north of the 6,000-point mark. This weekend, analysts, investors and traders will all be asking themselves what has happened since early July."
Prices of gold, which is traditionally seen as a safe haven in times of economic crisis, held firm as the dollar fell against the euro after the US jobs boost.
But Kingsview Financial head trader and strategist Matt Zeman said: "Even gold is susceptible. People are pretty much getting out of everything, except cash and bonds."
Investor confidence was hit on Thursday when EU president Jose Manuel Barroso wrote a letter to eurozone members, warning the debt crisis was spreading.