(CNN) -- Stocks on key Asian exchanges dropped modestly early Monday on what is likely to be an eventful day in world markets, following Standard and Poor's downgrade of the U.S. credit rating.





In early Tokyo trading, the Nikkei index fell 124 points, or 1.3%.





South Korea's KOSPI index slipped 1.6%. In Australia, the All Ordinaries index lost 0.7%. The Shanghai composite started 0.8% lower.





Hong Kong's Hang Seng index tumbled 2.6% at the open.





Similarly, U.S. stock futures fell around 1.7% in early electronic trading Sunday.





The futures were the first U.S. gauge of investor sentiment following Friday night's downgrade, removing the United States' AAA status for the first time. They give an indication of how investors will react when regular-hours U.S. trading begins at 9:30 a.m. ET Monday.





Besides the U.S. downgrade, investors are concerned about the debt crisis in some European nations, though actions on the part of the G7 and the European Central Bank Sunday helped to allay some of those fears.





Financial representatives of leading industrial nations said they are committed to taking "all necessary measures to support financial stability and growth in a spirit of close cooperation and confidence."





They welcomed the "decisive actions taken in the U.S. and Europe" and "the additional policy measures announced by Italy and Spain to strengthen fiscal discipline and underpin the recovery in economic activity and job creation."





"We are committed to taking coordinated action where needed, to ensuring liquidity, and to supporting financial market functioning, financial stability and economic growth," G7 finance ministers and central bank governors said in a Sunday night statement.





Treasury Secretary Tim Geithner had been expected to take part in a conference call with representatives of the other G7 nations to discuss the downgraded U.S. credit rating, a G7 official told CNN.





The G7 nations are the United Kingdom, France, Germany, Italy, Japan, Canada and the United States.





Similarly, the European Central Bank made a bid to calm markets Sunday. It said it would implement a bond-purchase program and welcomed announcements by Italy and Spain on new measures meant to reduce their deficits. It told the governments of those countries that a "decisive and swift implementation" of reforms is "essential."





The move represents an escalation in the official response to Europe's debt crisis, which is now more than a year old and until recently was contained to smaller economies like Greece, Ireland and Portugal.





International Monetary Fund Managing Director Christine Lagarde cheered the announcements.





"I welcome the statements from the European Central Bank, from the leaders of Germany and France as well as from the G7, and their renewed commitment to take all necessary action in a coordinated way to ensure stability and liquidity in the financial markets. This cooperation will contribute to maintaining confidence and spurring global economic growth," she said in a statement.





Middle Eastern markets, the first to open since the downgrade, were sharply lower on Sunday. Israel's market temporarily halted trading at one point and finished down more than 6%, while the Dubai Financial Market General Index fell 3.7%.





The General Index on the Abu Dhabi Securities Exchange was down more than 2.5%, while in Saudi Arabia, the Tadawul All-Share Index dropped nearly 5.5% in trading Saturday.





U.S. officials are talking to a "wide range of investors" about the downgrade by the credit agency to try to "mitigate" any short-term negative impact from Friday's announcement, a Treasury official told CNN.





Top Standard & Poor's officials said Sunday that the downgraded credit rating for the United States was both a call for political consensus on significant deficit reduction and a warning of possible further credit problems down the road.





"We have a negative outlook on the rating and that means that we think the risks currently on the rating are to the downside," said David Beers, the S&P global head of sovereign ratings, on "Fox News Sunday."





However, Beers said markets were reacting to the debt crises in some European countries and fears of a global
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