GLOBAL MARKETS-Stocks, euro tumble on debt fears, US outlook cut

* Potential Greek debt restructuring in focus

* S&P revises its credit outlook on U.S. to negative

* Finnish vote adds concerns about euro zone bailouts (Updates prices, adds details, comment, adds byline)

By Wanfeng Zhou

NEW YORK, April 18 (Reuters) - Fresh worries over Europe's debt crisis and a downgrade of the U.S. credit outlook by Standard & Poors spurred a sell off in major world stockmarkets on Monday.

Fresh fears that Greece will have to restructure its debt possibly as early as the summer, along with uncertainty over Portugal's pending bailout by the European Union exacerbated anxiety over how European policy-makers will handle the region's festering debt and bank problems.

Rating agency Standard & Poor's revised its outlook on the United States to negative from stable, citing a "material risk" that policymakers may not reach agreement on a plan to trim its large budget deficit. See [ID:nN18195555]

While the agency maintained the country's top AAA credit rating, it said the move signals there's at least a one-in-three chance that it could cut its long-term rating on the United States within two years.

"On a day when sovereign debt troubles have returned to haunt the euro, S&P's announcement added salt to the wound," said Kathy Lien, director of research at GFT in New York. "Investors were risk averse going into the NY open and will now remain cautious or nervous throughout the North American trading session."

MSCI's all-country world stock index <.MIWD00000PUS> started what is in many places a holiday-shortened week by losing 1.8 percent.

Wall Street stocks tumbled. The Dow Jones industrial average <.DJI> dropped 200.45 points, or 1.63 percent, to 12,141.57. The Standard & Poor's 500 Index <.SPX> dropped 19.76 points, or 1.50 percent, to 1,300.15. The Nasdaq Composite Index <.IXIC> dropped 47.95 points, or 1.74 percent, to 2,716.38.

The euro hit a two-week low against the U.S. dollar of $1.4155 on Reuters data, briefing paring losses after the S&P's announcement. It last traded down 1.6 percent at $1.4193, on track for its biggest one-day drop since late November. The euro also lost 2.5 percent to 116.96 yen .

German government sources told Reuters in Berlin that they did not believe Greece, which sealed a 110 billion euro ($157.7 billion) bailout from the EU and IMF last year, would make it through the summer without restructuring.

A restructuring of Greek debt would be the first by a west European nation in over half a century. The Greek government has denied repeatedly that it plans to restructure. See [ID:nLDE73H0IC]

Pressure on Portugal also grew after the anti-euro True Finns party scored big gains in the Sunday vote and vowed immediately to push for changes to a Portuguese rescue that is expected to total 80 billion euros when it is finalised by a mid-May deadline.

The U.S. dollar lost 0.8 percent to 82.42 yen, hitting a session low of 82.16 yen, the weakest in about two weeks.

Avery Shenfeld, chief economist at CIBC World Markets in Toronto, said the S&P announcement is really only catching up to "what markets have already priced in.

"The prospect of an actual default by the U.S. on debt issued in its own currency isn't a realistic worry, in a financial market that has a lot more real worries to deal with, including genuine Eurozone default risks," he said.

"We are less concerned over a downgrade to the outlook than we are about the growth implications of turning to fiscal belt tightening before the economy has self-sustaining momentum."

In Europe the FTSEurofirst 300 <.FTEU3> was off 1.6 percent.

GOLD HITS RECORD

Gold shot to record highs of nearly $1,500 an ounce while other commodities tumbled as investors fled to safe-haven assets after the S&P's move.

Spot gold , which tracks trades in bullion, rose to an all-time high of $1,497.20 an ounce. Benchmark gold futures in New York rose to a record of $1,498.60 an ounce. [GOL/]

"The U.S. debt situation got a reality check this morning from the move by S&P," said John Kilduff, partner at Again Capital in New York. "Only precious metals will be seen as attractive in the aftermath of the outlook downgrade."

Oil came under pressure after top exporter Saudi Arabia saying weak demand had forced it to reduce crude output. Brent crude for June fell $2.28 to $121.17 a barrel, having slipped as low as $121.00. U.S. crude for May fell $2.78 to $106.88, having slipped as low as $106.59.

In the bond market, longer-dated U.S. Treasury debt prices fell, but benchmark 10-year Treasury yields touched the lowest in over three weeks on worries about Europe's debt crisis.

The yield on the 30-year Treasury bond , which rises when there are jitters about U.S. credit-worthiness, rose 5 basis points to 4.497 percent. For more, see [US/] (Reporting by Richard Leong Chuck Mikolajczak, Barani Krishnan, Robert Gibbons