Top economic officials from around the world scrambled Sunday to contain the fallout from an unprecedented downgrade of the U.S. credit rating and a serious worsening of Europe’s economy.

Treasury Secretary Timothy F. Geithner and Federal Reserve Chairman Ben S. Bernanke joined counterparts from six of the world’s largest economies in an emergency conference call Sunday evening to discuss how world markets would respond to the Standard & Poor’s downgrade and the escalating European debt crisis. Afterward the officials released a statement pledging to support financial stability.

At the same time, the European Central Bank, after a hastily arranged meeting, signaled that it would invest in European bond markets in a bid to prop up hard-hit Italy and Spain, Europe’s fourth- and fifth-largest economies, which are in the midst of a worsening financial crisis.

The ECB intervention at first appeared to buoy markets Monday, driving down borrowing costs for both Italy and Spain and sending stocks in Milan and Madrid sharply higher.

But markets in London, Germany and elsewhere were dropping, with losses ranging between 1.6 and 2.9 percent.

Asian markets also sank, with Japan’s Nikkei closing at its lowest mark since mid-March. South Korea’s exchange halted traded briefly amid steep losses, and ultimately closed down 3.82 percent.

U.S. and European officials are trying to calm anxiety about the global economy as the U.S. downgrade and European debt problems threaten to feed on each other, weighing on markets and a limp economy on both sides of the Atlantic.

The emergency actions Sunday evoked memories of the response to the financial crisis in 2008 and portended intense volatility in global financial markets this week. The dollar fell over the weekend while gold soared. Stock markets in the Middle East plunged, and U.S. stock futures appeared negative. Asian markets dropped at the opening.

After the emergency conference call involving Geithner and Bernanke, the top seven economies expressed support for actions taken by both the United States and Europe and committed “to taking coordinated action where needed, to ensuring liquidity, and to supporting financial market functioning, financial stability and economic growth.” They said they would particularly take action to curb volatility in currency trading.

Amid this uncertainty, the Obama administration announced Sunday that Geithner, the president’s longest-serving economic adviser, would remain in his post through fall 2012. Geithner had told President Obama that he was ready to step down after leaders reached an agreement to raise the debt ceiling last week, but the president asked him to stay.

Geithner told Obama on Friday morning that he would agree to remain in the administration — only to inform the president later in the afternoon that the country would face a downgrade.

S&P cited the U.S. debt burden and political paralysis in its decision to remove the nation’s sterling AAA rating. The Obama administration blasted the decision, saying it was based on faulty logic and math, while acknowledging that Washington must do more to tame its debt.

“I think S&P showed really terrible judgment,” Geithner said Sunday on NBC’s “Nightly News.” “Our country is much stronger than Washington.”

The political parties continued to point fingers at each other Sunday for the downgrade. On NBC’s “Meet the Press,” Sen. John F. Kerry (D-Mass.) called S&P’s action the “tea party downgrade,” referring to conservative Republicans who refused to vote for an increase in the federal debt limit.

On CBS’s “Face the Nation,’’ Sen. Lindsey O. Graham (R-S.C.) blamed Obama, saying that “if he was in the Southeastern Conference, he’d be fired as a coach.”

Most analysts say the markets have been prepared for a downgrade by S&P, which had been issuing threats about a downgrade for months. And although the direct impact might be limited, the downgrade adds to broader concern about the state of the U.S. and European economies.

On Monday, S&P will issue more detailed guidance about the impact of the downgrade on the many entities whose own ratings rely on the U.S. government’s AAA rating. These include money market funds, government-owned corporations such as Fannie Mae and Freddie Mac, banks, insurance firms, and states and localities, including those in the Washington area.

The ratings on numerous municipalities are likely to be downgraded after the S&P action, according to a report issued Saturday by J.P. Morgan Chase.

Meanwhile, on Tuesday, the Fed is set to meet amid increasing evidence that the U.S. economic recovery is faltering.

The central bank is expected to downgrade its assessment of the U.S. economy and is considering taking new, modest steps to bolster economic growth, such as pledging to continue its ongoing efforts to support the economy for a longer period. Fed officials are unlikely to take a significant step toward stimulating the economy at this meeting.

By intervening in bond markets, the European Central Bank could at least temporarily take some of the pressure off Italy and Spain. Investors have been dumping Spanish and Italian bonds, driving their borrowing costs to record levels in recent days.

The events have sparked fears that the world’s seventh- and 12th-largest economies could be engulfed by the same kind of crisis that forced far smaller Greece, Ireland and Portugal to request emergency bailouts.

The ECB, as is customary, did not explicitly say it would buy Italian and Spanish bonds. But it strongly suggested that it would. The bank’s governing council agreed after an eleventh-hour emergency teleconference to take more drastic steps “to ensure [bond] price stability in the euro area.”

Raj Badiani, economist with IHS Global Insight in London, said the ECB’s action represents “a vote of confidence in the respective governments and an attempt to provide a sharp jolt to the negative sentiment engulfing Spain and Italy.”

German Chancellor Angela Merkel and French President Nicolas Sarkozy issued a separate statement Sunday saying that they support efforts announced last week by Spain and Italy to shore up their finances and that they support an expanded European rescue fund.

But analysts say the actions may be only a short-term solution. If Italy or Spain fail to quell market panic, analysts say, Europeans might be forced to move toward the advent of a new euro-bond, putting the economic weight of Germany and France behind their profligate neighbors. Germany and other strong European economies are opposed to such a deal or other measures that would make them carry the burden of their struggling neighbors any longer.

Concern over Italy, in particular, is growing in large part because Europe’s economies are slowing at the same time that the countries need strong economic growth to tame their debt loads.

Although a snapshot of the Italian economy released by authorities Friday indicated that the country’s economic growth is better than that of some other nations in the region, investors are fixating on its $2.2 trillion debt and the question of who would aid Italy if it cannot pay its bills.

Last week, the ECB began to scoop up the debt of Portugal and Ireland for the first time since March. But even that measure was opposed by Germany and other fiscally conservative nations, and analysts expect greater concern about the vast sums that would be spent to bolster Italy and Spain.

In a worst-case scenario, Italy would need a bailout of about $1.4 trillion — or more than double the size of an established European rescue fund.

Faiola wrote from London. Staff writers Cezary Podkul and Neil Irwin contributed to this report.